fix(#792): guard fleet roster schema errors

docs/scratchpads/ms-792-fleet-enoent-installer.md

@@ -20,6 +20,7 @@ Make expected missing or malformed fleet roster configuration fail with an actio
 - 2026-07-16: Replaced installer heading `echo` with format-safe `printf`; added a regression check for the scoped package heading.
 - 2026-07-16: Rebuilt CLI and manually verified `fleet ps` with no roster prints the initialization hint, exits 1, and has no stack trace.
 - 2026-07-17: Rebased #818 onto `origin/main` at `9ddc6fbd` (#791 PR3). The added `fleet regen` command had a canonical roster read in its sibling module; it now uses the same missing-roster guard and Commander exit path. Internal NORTH_STAR, preset, and post-write invariant reads remain intentionally unguarded.
+- 2026-07-17: RoR found that semantically invalid v1 documents still escaped as plain `Error` values. `normalizeFleetRosterV1` now preserves each validation message while converting it to `FleetRosterConfigurationError`, so its command callers use the actionable nonzero Commander path.
 
 ## Verification
 
@@ -29,6 +30,7 @@ Make expected missing or malformed fleet roster configuration fail with an actio
 - `pnpm format:check` — PASS
 - `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts src/commands/install-heading.spec.ts` — PASS (209 tests)
 - `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet-regen-command.spec.ts` — PASS (27 tests, including missing canonical roster)
+- `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts -t "semantically invalid v1 roster"` — RED then PASS; verifies duplicate agent names are reported as `fleet.roster` exit 1 without a stack trace.
 - Instrumented Vitest coverage is unavailable because `@vitest/coverage-v8` is not declared in this repository. Each branch added in the roster guard has direct unit coverage.
 
 ## Risks / blockers

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

@@ -214,6 +214,42 @@ describe('fleet roster configuration failures', () => {
       'Fleet roster at /srv/mosaic/fleet/roster.yaml is invalid. Fix the file or run `mosaic fleet init --force`.',
     );
   });
+
+  it('reports a semantically invalid v1 roster as an actionable nonzero command error', async () => {
+    const home = await tempDir();
+    const rosterPath = join(home, 'fleet', 'roster.yaml');
+    const stderrSpy = vi.spyOn(process.stderr, 'write').mockImplementation(() => true);
+    try {
+      await mkdir(dirname(rosterPath), { recursive: true });
+      await writeFile(
+        rosterPath,
+        [
+          'version: 1',
+          'transport: tmux',
+          'agents:',
+          '  - name: canary-pi',
+          '    runtime: pi',
+          '  - name: canary-pi',
+          '    runtime: codex',
+        ].join('\n'),
+      );
+
+      await expect(
+        buildProgram().parseAsync(['node', 'mosaic', 'fleet', '--mosaic-home', home, 'ps']),
+      ).rejects.toMatchObject({
+        code: 'fleet.roster',
+        exitCode: 1,
+        message: 'Fleet roster has duplicate agent name: canary-pi.',
+      });
+      expect(stderrSpy.mock.calls.flat().join('')).toContain(
+        'Fleet roster has duplicate agent name: canary-pi.',
+      );
+      expect(stderrSpy.mock.calls.flat().join('')).not.toMatch(/\n\s+at\s/);
+    } finally {
+      stderrSpy.mockRestore();
+      await rm(home, { recursive: true, force: true });
+    }
+  });
 });
 
 describe('fleet roster parsing', () => {

packages/mosaic/src/fleet/fleet-roster-v1.ts

@@ -197,6 +197,16 @@ export function getRosterAgent(roster: FleetRoster, name: string): FleetAgent {
 }
 
 export function normalizeFleetRosterV1(raw: RawFleetRoster): FleetRoster {
+  try {
+    return normalizeFleetRosterV1Unchecked(raw);
+  } catch (error) {
+    if (error instanceof FleetRosterConfigurationError) throw error;
+    if (error instanceof Error) throw new FleetRosterConfigurationError(error.message);
+    throw error;
+  }
+}
+
+function normalizeFleetRosterV1Unchecked(raw: RawFleetRoster): FleetRoster {
   assertObject(raw, 'Fleet roster');
   assertKnownKeys(raw, 'Fleet roster', [
     'version',