refactor(fleet): rename tmux socket mosaic-factory → mosaic-fleet

Pure rename of the named production-isolation socket to match the product brand
(Mosaic Fleet), per Jason. Behavior is identical — only the socket NAME changes.

- 6 example presets: socket_name: mosaic-factory → mosaic-fleet
- fleet.ts: DEFAULT_SOCKET_NAME = 'mosaic-fleet' (+ all literals)
- systemd units + READMEs, roster.schema.json, start-agent-session.sh
- docs/guides + fleet PRD/TASKS + scratchpads; tests updated

The PoC roster is socket-LESS (default socket, no -L), so it is unaffected.
docs/fleet/north-star.md is intentionally EXCLUDED (owned by the in-flight
doctrine PR #629 — its mosaic-factory references are renamed there to avoid a
merge conflict). The live legacy canary fleet still running on the old socket is
a separate retire/migrate op, not this PR.

Single find/replace — target trivially swappable if the brand is reconsidered.

Verified: 172 fleet + onboarding tests green; tsc/eslint/prettier/sanitize clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

docs/scratchpads/fleet-observability-phase2.md

@@ -31,7 +31,7 @@ with a second agent on `dragon-lin`.
 ## Environment facts (verified 2026-06-20)
 
 - Fleet is live on `W-jarvis` (uid 1000, `jarvis`, `Linger=yes`) on tmux socket
-  `mosaic-factory`: `_holder`, `canary-pi`, `dogfood-coder`, `dogfood-orchestrator`,
+  `mosaic-fleet`: `_holder`, `canary-pi`, `dogfood-coder`, `dogfood-orchestrator`,
   `dogfood-reviewer`. All panes run `~/.config/mosaic/fleet/dogfood-agent.py` (stub),
   including `canary-pi` (roster says runtime=pi → **drift**).
 - Holder + `mosaic-agent@*` units are `active (exited)` but `UnitFileState=disabled`
@@ -56,7 +56,7 @@ with a second agent on `dragon-lin`.
   with dragon-lin coder, commit docs, begin Phase-2 delivery (heartbeat + `fleet ps`).
 - 2026-06-20 (session 2): Built Phase-2 CLI via worker (commit ab47831): `fleet ps`,
   `agent watch`, `agent send --verify`, 62 tests. LIVE-verified `fleet ps` on
-  mosaic-factory — correctly flagged canary-pi DRIFT + BOOT-ENABLE, tenant_id+host in JSON.
+  mosaic-fleet — correctly flagged canary-pi DRIFT + BOOT-ENABLE, tenant_id+host in JSON.
   Heartbeat responder added to dogfood-agent.py (FLEET-OBS-002) — `fleet ps` HB now
   `healthy` for all 4 agents.
 - Coordination: dual-engine-reviewed (Claude+Codex) and merged framework PRs #572