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/systemd/user/README.md

@@ -33,7 +33,7 @@ Per-agent overrides live outside the package in:
 Example:
 
 ```dotenv
-MOSAIC_TMUX_SOCKET=mosaic-factory
+MOSAIC_TMUX_SOCKET=mosaic-fleet
 MOSAIC_AGENT_RUNTIME=claude
 MOSAIC_AGENT_WORKDIR=$HOME/src/your-project
 # Optional escape hatch for PoC/canary agents:
@@ -50,8 +50,8 @@ chmod +x ~/.config/mosaic/tools/fleet/start-agent-session.sh
 systemctl --user daemon-reload
 systemctl --user start mosaic-tmux-holder.service
 systemctl --user start mosaic-agent@canary.service
-tmux -L mosaic-factory ls
+tmux -L mosaic-fleet ls
 ```
 
-Do not use `tmux kill-server` without `-L mosaic-factory`; this pattern is meant
+Do not use `tmux kill-server` without `-L mosaic-fleet`; this pattern is meant
 to avoid disturbing the user's default tmux server.