fix(fleet): consume model_hint + fix socket-default trap (stand-up fixes) (#627)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

docs/scratchpads/fleet-standup-fixes.md

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+# Fleet stand-up fixes — model_hint→--model + socket-default trap (#626)
+
+- **Issue:** #626 · **Branch:** `feat/fleet-standup-fixes` (off main). PoC-blocking, before doctrine doc.
+
+## FIX 1 — model_hint consumed
+
+- generateAgentEnv emits `MOSAIC_AGENT_MODEL=<modelHint>` (bare empty when unset).
+- start-agent-session.sh default command → `mosaic yolo $RUNTIME ${MOSAIC_AGENT_MODEL:+--model $MOSAIC_AGENT_MODEL}`.
+  → pi workers launch with `--model openai-codex/gpt-5.5:high`.
+
+## 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).
+- `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
+  socket-less rosters (the PoC) get default-socket behavior. DEFAULT_SOCKET_NAME exported for explicit use.
+
+## Verification
+
+- 158 fleet + 201 fleet-adjacent tests green; new: socketArgs none/named, model_hint→env, explicit-socket
+  renders -L, socket-less env bare. tsc/eslint/prettier/sanitize clean. Shell bash -n + end-to-end sim
+  (socket-less→no -L, model→--model).