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/framework/install.sh

@@ -30,10 +30,12 @@ INSTALL_MODE="${MOSAIC_INSTALL_MODE:-prompt}"
 # own fleet files MUST
 # survive `mosaic update` (which runs this sync automatically): the active
 # roster (`fleet/roster.yaml` + any other `fleet/*.yaml`), per-agent env
-# (`fleet/agents/`), and heartbeat run dir (`fleet/run/`). Without these, an
-# update wipes the operator's fleet. Glob entries are honored by both the rsync
-# path (`--exclude`) and the glob-aware cp fallback below.
-PRESERVE_PATHS=("CONSTITUTION.md" "AGENTS.md" "SOUL.md" "USER.md" "TOOLS.md" "STANDARDS.md" "memory" "sources" "credentials" "fleet/*.yaml" "fleet/agents" "fleet/run")
+# (`fleet/agents/`), heartbeat run dir (`fleet/run/`), and the Mosaic-native
+# backlog-of-record store (`fleet/backlog/` — embedded PGlite data dir; see
+# packages/mosaic/src/commands/fleet-backlog.ts). Without these, an update
+# wipes the operator's fleet AND their backlog. Glob entries are honored by
+# both the rsync path (`--exclude`) and the glob-aware cp fallback below.
+PRESERVE_PATHS=("CONSTITUTION.md" "AGENTS.md" "SOUL.md" "USER.md" "TOOLS.md" "STANDARDS.md" "memory" "sources" "credentials" "fleet/*.yaml" "fleet/agents" "fleet/run" "fleet/backlog")
 
 # Framework-owned contract files: re-copied from defaults/ on every upgrade (the
 # user must not edit them; a divergent copy is backed up once before overwrite).