feat(fleet): native Mosaic backlog on @mosaicstack/db (atomic claim + TTL)

Add Mosaic's own backlog-of-record on the existing Postgres storage layer,
replacing the former Hermes adapter (no Hermes dependency) and the earlier
sqlite idea (no sqlite, no new client).

- packages/db: `backlog` table (drizzle) + enum + migration 0011; BacklogService
  taking a Db handle so it runs on both createDb (server PG) and createPgliteDb
  (embedded). Atomic claim via SELECT ... FOR UPDATE SKIP LOCKED inside one tx,
  TTL claims + reclaim of expired, deps DAG gate, idempotency-key dedup, stats.
- packages/mosaic: `mosaic fleet backlog <sub> --json` (create/list/claim/
  reclaim/link/stats/block/complete). Embedded PGlite default at
  <mosaicHome>/fleet/backlog; full Postgres via DATABASE_URL — same code.
  Migrations run on first use.
- install.sh: preserve fleet/backlog across `mosaic update`.
- docs/fleet/backlog-conventions.md: schema, phase convention, atomic-claim +
  TTL semantics, PGlite-default/Postgres-by-config, Mosaic-native (no Hermes).
- Tests (in-memory PGlite, real PG semantics): create/list, exactly-one-winner
  concurrency (and N-card no-double-claim), deps gate, reclaim of expired, stats.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -78,6 +78,7 @@ describe('registerFleetCommand', () => {
     expect(fleet).toBeDefined();
     expect(fleet!.commands.map((command) => command.name()).sort()).toEqual([
       'add',
+      'backlog',
       'init',
       'install',
       'install-systemd',
@@ -91,6 +92,24 @@ describe('registerFleetCommand', () => {
     ]);
   });
 
+  it('registers the backlog subcommand with its operations', () => {
+    const program = buildProgram();
+    const fleet = program.commands.find((command) => command.name() === 'fleet');
+    const backlog = fleet!.commands.find((command) => command.name() === 'backlog');
+
+    expect(backlog).toBeDefined();
+    expect(backlog!.commands.map((command) => command.name()).sort()).toEqual([
+      'block',
+      'claim',
+      'complete',
+      'create',
+      'link',
+      'list',
+      'reclaim',
+      'stats',
+    ]);
+  });
+
   it('adds fleet-backed agent subcommands without removing existing options', () => {
     const program = buildProgram();
     const agent = program.commands.find((command) => command.name() === 'agent');