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

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 AGENT_NAME=${1:-${MOSAIC_AGENT_NAME:-}}
 # Absent socket ⇒ the LITERAL default tmux socket (no -L). The roster's
-# socket_name is honored when set; absent never silently becomes mosaic-factory
+# socket_name is honored when set; absent never silently becomes mosaic-fleet
 # (spawn stays consistent with the onboarding cheat-sheet + fleet ps observe).
 MOSAIC_TMUX_SOCKET=${MOSAIC_TMUX_SOCKET:-}
 MOSAIC_AGENT_RUNTIME=${MOSAIC_AGENT_RUNTIME:-pi}