fix(fleet): consume model_hint + fix socket-default trap (#626)

Two spawn-side blockers found building the live PoC roster.

FIX 1 — model_hint not consumed: start-agent-session.sh built 'mosaic yolo
$RUNTIME' with no --model, so pi workers ignored the roster's model. Now
generateAgentEnv emits MOSAIC_AGENT_MODEL=<hint> and the launcher appends
${MOSAIC_AGENT_MODEL:+--model $MOSAIC_AGENT_MODEL} → workers run on e.g.
openai-codex/gpt-5.5:high.

FIX 2 — socket default trap: an ABSENT roster socket silently became
mosaic-factory in THREE places (parseRosterText fallback; the
mosaic-agent@.service Environment= default + ExecStop :-mosaic-factory;
start-agent-session :-mosaic-factory). The live PoC runs on the DEFAULT tmux
socket (socket_name absent). Now absent ⇒ '' ⇒ the literal default socket (no
-L) consistently across spawn, the systemd unit, fleet ps/watch observe, and
the onboarding cheat-sheet:
- socketArgs(name) → name ? ['-L', name] : []; replaces all ~15 -L sites in
  fleet.ts. parseRosterText fallback '' (was DEFAULT_SOCKET_NAME).
- shellEnvValue('') now emits a BARE 'VAR=' (not ''), so a socket-less .env can
  never yield a literal socket named "''" under systemd EnvironmentFile.
- start-agent-session.sh _tmux wrapper passes -L only when a socket is set;
  mosaic-agent@.service drops the socket default + uses a conditional ExecStop.

CONTAINMENT: all 6 shipped presets set socket_name: mosaic-factory explicitly,
so they are unaffected — only socket-less rosters (the PoC) get default-socket
behavior. DEFAULT_SOCKET_NAME exported as a constant for explicit isolation.

Verified: 158 fleet + 201 fleet-adjacent tests green (socketArgs none/named,
model_hint→env, explicit-socket renders -L, socket-less bare env); shell bash -n
+ end-to-end sim (socket-less→no -L, model→--model); tsc/eslint/prettier/
sanitize clean.

Refs #626

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

packages/mosaic/framework/systemd/user/mosaic-agent@.service

@@ -8,13 +8,15 @@ PartOf=mosaic-tmux-holder.service
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+# No default MOSAIC_TMUX_SOCKET: an absent roster socket means the literal
+# default tmux socket (no -L). The per-agent .env sets it when the roster names
+# one; otherwise it stays unset and start-agent-session.sh uses the default socket.
 Environment=MOSAIC_AGENT_NAME=%i
 Environment=MOSAIC_AGENT_RUNTIME=pi
 Environment=MOSAIC_AGENT_WORKDIR=%h
 EnvironmentFile=-%h/.config/mosaic/fleet/agents/%i.env
 ExecStart=/bin/bash %h/.config/mosaic/tools/fleet/start-agent-session.sh %i
-ExecStop=-/bin/bash -lc 'tmux -L "${MOSAIC_TMUX_SOCKET:-mosaic-factory}" kill-session -t "=%i"'
+ExecStop=-/bin/bash -lc 'if [ -n "${MOSAIC_TMUX_SOCKET:-}" ]; then tmux -L "$MOSAIC_TMUX_SOCKET" kill-session -t "=%i"; else tmux kill-session -t "=%i"; fi'
 
 [Install]
 WantedBy=default.target