fleet: move directories off managed paths instead of refusing forever

Launch will not delete a real directory sitting where it expects a managed
link -- an auth/<harness>/primary that someone logged into by hand, or a
plugin directory a seat acquired before the central store existed. That
refusal is right and it is also a dead end: the operator gets a composition
error and no way forward.

`mosaic fleet adopt` is the way forward. Bare, it lists every such directory
and the command that resolves it. With a verb, it moves one where it belongs.

Nothing here deletes. A promotion is a rename; an occupied destination is a
refusal, not a merge; a cross-device rename is reported rather than retried as
copy-then-delete, because a copy-then-delete is a delete.

Store adoption stops at the move and does not install the link. The seat's
.mosaic-managed-links.json belongs to launch, and a link written behind it
fails the next composition as an unrecorded symlink -- one refusal traded for
another. The next launch installs and records it when the profile lists the
entry; whether a seat gets a plugin stays `mosaic fleet plugin`'s decision.

W-F3 of docs/plans/2026-08-14_fleet-seats-on-web1.md.

packages/mosaic/src/commands/fleet.ts

@@ -42,6 +42,7 @@ import {
   registerFleetAgentScaffoldCommand,
   type FleetAgentScaffoldCommandDeps,
 } from './fleet-agent-scaffold-command.js';
+import { registerFleetAdoptCommand } from './fleet-adopt-command.js';
 import {
   registerFleetMigrationCommand,
   type FleetMigrationCommandDeps,
@@ -2080,6 +2081,11 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
     ...(deps.fleetDataHome === undefined ? {} : { fleetDataHome: deps.fleetDataHome }),
     mosaicHomeFor: () => cmd.opts<{ mosaicHome: string }>().mosaicHome,
   });
+  // The counterpart to launch's refusals: launch will not delete a real directory sitting on
+  // a managed path, and this is how one gets moved out of the way instead.
+  registerFleetAdoptCommand(cmd, {
+    ...(deps.fleetDataHome === undefined ? {} : { fleetDataHome: deps.fleetDataHome }),
+  });
   // Roster-v2 desired-state mutations belong directly to the fleet control
   // plane; they do not share the root `mosaic agent` gateway-backed surface.
   registerFleetAgentCrudCommands(cmd, deps);