mosaic watch: first embedded brain-tool dispatch command

Architecture (fleet CLI integration, Jason ruling 2026-08-28): the
package embeds the command surface; operator-owned implementations stay
in the brain (tools/agent-watch). The command resolves the brain home
via resolveBrainHome (MOSAIC_BRAIN_HOME wins, canonical ~/.mosaic
adoption otherwise) and execs the tool there.

- Pass-through contract: allowUnknownOption + variadic args capture the
  full ordered argument list (commander 13 measured behavior; no
  passThroughOptions, which would force enablePositionalOptions on the
  root program fleet-wide). Args, stdout/stderr, and the exit code
  belong to the tool.
- Absent tool: exit 127 naming the resolved path (no guessing).
- Signal death / null spawn status: exit 125.
- Spec (6/6 green): brain-home precedence, exit mapping, absent-tool
  path. Live smoke verified through the fleet launcher: mosaic watch list
  returns real watcher state with exit-code passthrough.

Pre-existing local vitest failures in fleet-agent-crud/fleet-regen/
compose-contract specs measured identical on a clean origin/next stash;
not caused by this change (PR #1462 CI was terminal-green).

packages/mosaic/src/cli.ts

@@ -11,6 +11,7 @@ import { registerQualityRails } from '@mosaicstack/quality-rails';
 import { registerQueueCommand } from '@mosaicstack/queue';
 import { registerStorageCommand } from '@mosaicstack/storage';
 import { registerTelemetryCommand } from './commands/telemetry.js';
+import { registerWatchCommand } from './commands/watch.js';
 import { registerAgentCommand } from './commands/agent.js';
 import { registerInteractionCommand } from './commands/interaction.js';
 import { registerConfigCommand } from './commands/config.js';
@@ -428,6 +429,7 @@ registerSkillCommand(program);
 // ─── telemetry ───────────────────────────────────────────────────────────────
 
 registerTelemetryCommand(program);
+registerWatchCommand(program);
 
 // ─── update ─────────────────────────────────────────────────────────────
 

packages/mosaic/src/commands/watch.spec.ts

@@ -0,0 +1,72 @@
+import { mkdirSync, mkdtempSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import { Command } from 'commander';
+import { afterEach, describe, expect, it } from 'vitest';
+
+import { exitStatusFor, registerWatchCommand, resolveAgentWatchTool } from './watch.js';
+
+// The dispatch command execs a real process with inherited stdio; the spec
+// covers the pure resolution and exit-mapping surfaces plus the absent-tool
+// path (which exits without spawning). Live pass-through is exercised by the
+// fleet smoke test against the real brain tool.
+
+describe('resolveAgentWatchTool', () => {
+  const saved = process.env['MOSAIC_BRAIN_HOME'];
+  afterEach(() => {
+    if (saved === undefined) delete process.env['MOSAIC_BRAIN_HOME'];
+    else process.env['MOSAIC_BRAIN_HOME'] = saved;
+  });
+
+  it('honors MOSAIC_BRAIN_HOME over the canonical brain', () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'watch-resolve-'));
+    process.env['MOSAIC_BRAIN_HOME'] = tmp;
+    expect(resolveAgentWatchTool('/nonexistent/mosaic-home')).toBe(
+      join(tmp, 'tools', 'agent-watch', 'agent-watch.sh'),
+    );
+  });
+
+  it('resolves inside the brain tools tree', () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'watch-resolve-'));
+    process.env['MOSAIC_BRAIN_HOME'] = tmp;
+    const tool = resolveAgentWatchTool(tmp);
+    expect(tool.endsWith(join('tools', 'agent-watch', 'agent-watch.sh'))).toBe(true);
+  });
+});
+
+describe('exitStatusFor', () => {
+  it('maps absent tool to 127', () => {
+    expect(exitStatusFor({ status: 0 }, false)).toBe(127);
+  });
+
+  it('passes the tool exit status through', () => {
+    expect(exitStatusFor({ status: 2 }, true)).toBe(2);
+    expect(exitStatusFor({ status: 78 }, true)).toBe(78);
+  });
+
+  it('maps signal death / null status to 125', () => {
+    expect(exitStatusFor({ status: null }, true)).toBe(125);
+  });
+});
+
+describe('registerWatchCommand absent-tool path', () => {
+  const saved = process.env['MOSAIC_BRAIN_HOME'];
+  afterEach(() => {
+    if (saved === undefined) delete process.env['MOSAIC_BRAIN_HOME'];
+    else process.env['MOSAIC_BRAIN_HOME'] = saved;
+    process.exitCode = undefined;
+  });
+
+  it('sets exitCode 127 with the resolved path when the tool is missing', async () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'watch-missing-'));
+    // The suite directory exists but the tool file does not.
+    mkdirSync(join(tmp, 'tools', 'agent-watch'), { recursive: true });
+    process.env['MOSAIC_BRAIN_HOME'] = tmp;
+
+    const program = new Command();
+    registerWatchCommand(program);
+    await program.parseAsync(['watch', 'list'], { from: 'user' });
+    expect(process.exitCode).toBe(127);
+  });
+});

packages/mosaic/src/commands/watch.ts

@@ -0,0 +1,87 @@
+import type { Command } from 'commander';
+import { spawnSync } from 'node:child_process';
+import { accessSync, constants } from 'node:fs';
+import { join } from 'node:path';
+
+import { DEFAULT_MOSAIC_HOME } from '../constants.js';
+import { resolveBrainHome } from '../fleet/brain-home.js';
+
+/**
+ * `mosaic watch` — dispatch to the brain's agent-watch suite.
+ *
+ * Architecture (fleet CLI integration, Jason ruling 2026-08-28): the npm
+ * package embeds the COMMAND SURFACE; operator-owned tool implementations
+ * stay in the brain (`~/.mosaic/tools/`). The dispatch resolves the brain
+ * home (MOSAIC_BRAIN_HOME wins, else canonical ~/.mosaic when adopted —
+ * see brain-home.ts) and execs the tool there. Nothing operator-specific
+ * ships inside the package, keeping the framework boundary clean.
+ *
+ * Pass-through contract: arguments, stdout/stderr, and the exit code belong
+ * to the tool. The CLI adds nothing on success. When the tool is absent the
+ * CLI fails loudly with the resolved path (exit 127) instead of guessing.
+ */
+
+export function resolveAgentWatchTool(mosaicHome: string): string {
+  return join(resolveBrainHome(mosaicHome), 'tools', 'agent-watch', 'agent-watch.sh');
+}
+
+/** Map a spawnSync result + tool existence to the CLI exit status. */
+export function exitStatusFor(
+  result: { status: number | null; error?: NodeJS.ErrnoException },
+  toolExists: boolean,
+): number {
+  if (!toolExists) return 127;
+  if (result.status !== null) return result.status;
+  return 125; // killed by signal / could not run
+}
+
+export function registerWatchCommand(program: Command): void {
+  const cmd: Command = program
+    .command('watch')
+    .description('Wake-me-when watchers (agent-watch): start, list, stop')
+    // allowUnknownOption + variadic = full ordered pass-through: unknown
+    // options (--name, --when, ...) and their values land in args verbatim
+    // (commander 13 measured behavior), so the tool owns its own flag
+    // surface without the CLI needing passThroughOptions (which would
+    // force enablePositionalOptions fleet-wide on the root program).
+    .allowUnknownOption()
+    .argument('[args...]', 'args passed through to agent-watch.sh')
+    .action(async (args: string[], _opts: unknown, command: Command) => {
+      // --mosaic-home is not global in this CLI; walk parents for it and
+      // fall back to the default. MOSAIC_BRAIN_HOME (seat launchers export
+      // it) wins inside resolveBrainHome regardless.
+      let mosaicHome: string | undefined;
+      for (let anc: Command | null = command; anc; anc = anc.parent) {
+        const v = (anc.opts() as Record<string, string | undefined>)['mosaicHome'];
+        if (v !== undefined) {
+          mosaicHome = v;
+          break;
+        }
+      }
+      const tool = resolveAgentWatchTool(mosaicHome ?? DEFAULT_MOSAIC_HOME);
+
+      let toolExists = true;
+      try {
+        accessSync(tool, constants.X_OK);
+      } catch {
+        toolExists = false;
+      }
+      if (!toolExists) {
+        console.error(
+          `mosaic watch: agent-watch not found (expected ${tool}). ` +
+            'The watcher suite lives in the brain tree under tools/agent-watch/; ' +
+            'check MOSAIC_BRAIN_HOME or the brain checkout.',
+        );
+        process.exitCode = 127;
+        return;
+      }
+
+      const result = spawnSync(tool, args, { stdio: 'inherit', env: process.env });
+      process.exitCode = exitStatusFor(result, true);
+    });
+
+  cmd.addHelpText(
+    'after',
+    '\nEverything after `mosaic watch` is passed through to agent-watch.sh verbatim (args, output, exit code).',
+  );
+}