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-standup-fixes.md

@@ -11,14 +11,14 @@
 ## FIX 2 — socket default trap (absent ⇒ literal default socket, no -L everywhere)
 
 - THE TRAP (3 sites): parseRosterText fallback was DEFAULT_SOCKET_NAME; systemd unit had
-  `Environment=MOSAIC_TMUX_SOCKET=mosaic-factory` + `ExecStop ${…:-mosaic-factory}`; start-agent-session
-  defaulted `:-mosaic-factory`. All fixed → absent socket = '' = default tmux socket (no -L).
+  `Environment=MOSAIC_TMUX_SOCKET=mosaic-fleet` + `ExecStop ${…:-mosaic-fleet}`; start-agent-session
+  defaulted `:-mosaic-fleet`. All fixed → absent socket = '' = default tmux socket (no -L).
 - `socketArgs(name)` helper → `name ? ['-L', name] : []`; replaced all ~15 -L render sites in fleet.ts.
 - shellEnvValue('') now emits a **bare** `VAR=` (not `''`) — unambiguous empty in systemd EnvironmentFile
   (a quoted '' could become a literal socket named "''").
 - start-agent-session.sh: `_tmux` wrapper passes -L only when socket set; mosaic-agent@.service: dropped the
   socket default + conditional ExecStop. So spawn == observe == onboarding cheat-sheet.
-- CONTAINMENT: all 6 shipped presets set socket_name: mosaic-factory explicitly → unaffected; only
+- CONTAINMENT: all 6 shipped presets set socket_name: mosaic-fleet explicitly → unaffected; only
   socket-less rosters (the PoC) get default-socket behavior. DEFAULT_SOCKET_NAME exported for explicit use.
 
 ## Verification