fleet: start a roster pane through its seat when one is scaffolded

The roster lane and the harness-homes lane did not touch. start-agent-session.sh
ran `mosaic yolo "$RUNTIME"` with HOME set to the operator's home, so every fleet
seat on a host shared the operator's harness home and, for Claude, the operator's
own ~/.claude credentials. Nothing in framework/ called `mosaic fleet launch` at
all, which meant ~/.mosaic was a directory nothing read.

The pane now runs `mosaic fleet launch "$AGENT_NAME"` when a scaffolded seat
exists at $PANE_HOME/.mosaic/fleet/agents/<name>/profile.json, and the historical
command otherwise. Detection uses $PANE_HOME/.mosaic rather than MOSAIC_DATA_HOME
because the pane environment is cleared with env -i; the composition resolves the
same root from HOME, so the two cannot disagree.

Additive by construction: a host with no scaffolded seats launches exactly as
before, so this can land ahead of any seat being enrolled.

- fleet launch gains --dangerous, threaded to launchFleetRuntime. Without it a
  seat launched from the roster would drop the permissions footing `mosaic yolo`
  gave it and prompt at a pane with nobody at it. The roster launcher asks for it
  explicitly so it stays visible in the process table instead of becoming a
  profile default.
- A caller's --model replaces the profile's instead of being appended after it.
  The roster carries a model per seat and is the surface operators edit; emitting
  both flags would leave the choice to each harness's argument parser.
- Claude workdir trust is written into the seat's .claude.json when the pane will
  run in a seat home. It previously always went to the operator's ~/.claude.json,
  which would leave the seat prompting on its first turn.

Covers Jason's scope amendment for web1: without this seam, "multiple
authentication accounts and agent pegging to auth" cannot be demonstrated on a
roster-managed seat.

packages/mosaic/src/commands/fleet-launch-command.ts

@@ -149,6 +149,7 @@ export interface FleetLaunchCommandDeps {
     args: string[],
     declaredEnv: Readonly<Record<string, string>>,
     context: FleetHarnessContext,
+    dangerous: boolean,
   ) => void;
 }
 
@@ -672,7 +673,10 @@ function buildArgv(
   passthrough: string[],
 ): string[] {
   const argv: string[] = [profile.harness];
-  if (profile.model) argv.push('--model', profile.model);
+  // A caller-supplied --model replaces the profile's rather than being appended after
+  // it. The fleet roster carries a model per seat and is the surface operators edit, so
+  // it has to win; emitting both flags would leave that to each harness's arg parser.
+  if (profile.model && !passthrough.includes('--model')) argv.push('--model', profile.model);
   if (profile.harness === 'pi') {
     for (const skill of profile.skills) argv.push('--skill', join(seatHome, 'skills', skill));
   }
@@ -963,33 +967,43 @@ export function registerFleetLaunchCommand(
     .command('launch <name>')
     .description('Compose and launch one per-agent harness home')
     .option('--dry-run', 'Print the fully resolved composition without writing or launching')
+    .option('--dangerous', 'Launch the seat in dangerous-permissions mode, as `mosaic yolo` does')
     .allowUnknownOption(true)
     .allowExcessArguments(true)
-    .action((name: string, opts: { dryRun?: boolean }, command: Command): void => {
-      try {
-        const userHome = deps.userHome ?? defaultFleetDataHome();
-        const passthrough = command.args.slice(1);
-        const plan = resolveFleetLaunchComposition(
-          name,
-          { systemHome: systemHomeFor(), userHome },
-          passthrough,
-        );
-        if (opts.dryRun === true) {
-          process.stdout.write(`${formatFleetLaunchDryRun(plan)}\n`);
-          return;
+    .action(
+      (name: string, opts: { dryRun?: boolean; dangerous?: boolean }, command: Command): void => {
+        try {
+          const userHome = deps.userHome ?? defaultFleetDataHome();
+          const passthrough = command.args.slice(1);
+          const plan = resolveFleetLaunchComposition(
+            name,
+            { systemHome: systemHomeFor(), userHome },
+            passthrough,
+          );
+          if (opts.dryRun === true) {
+            process.stdout.write(`${formatFleetLaunchDryRun(plan)}\n`);
+            return;
+          }
+          applyFleetLaunchComposition(plan);
+          console.log(`[mosaic] bundle: ${plan.bundle.display}`);
+          const launcher = deps.launcher ?? launchFleetRuntime;
+          // Dangerous mode is the caller's to ask for, not the seat's to assume. An
+          // unattended tmux seat needs it -- a permission prompt with nobody at the pane
+          // is a hung agent -- so the roster launcher passes the flag explicitly and it
+          // stays visible in the process table rather than hiding in a profile default.
+          launcher(
+            plan.profile.harness,
+            plan.argv.slice(1),
+            plan.env,
+            { agentDir: plan.agentDir, mosaicHome: plan.systemHome },
+            opts.dangerous === true,
+          );
+        } catch (error: unknown) {
+          process.exitCode = 1;
+          const code = error instanceof FleetLaunchError ? `${error.code}: ` : '';
+          const message = error instanceof Error ? error.message : String(error);
+          process.stderr.write(`mosaic fleet launch failed: ${code}${message}\n`);
         }
-        applyFleetLaunchComposition(plan);
-        console.log(`[mosaic] bundle: ${plan.bundle.display}`);
-        const launcher = deps.launcher ?? launchFleetRuntime;
-        launcher(plan.profile.harness, plan.argv.slice(1), plan.env, {
-          agentDir: plan.agentDir,
-          mosaicHome: plan.systemHome,
-        });
-      } catch (error: unknown) {
-        process.exitCode = 1;
-        const code = error instanceof FleetLaunchError ? `${error.code}: ` : '';
-        const message = error instanceof Error ? error.message : String(error);
-        process.stderr.write(`mosaic fleet launch failed: ${code}${message}\n`);
-      }
-    });
+      },
+    );
 }