fix(plugin): allow cold-start overlay launch without running process

docs/subagents/agents/codex-plugin-start-overlay-gate-20260223T044347Z-k3n8.md

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+# Agent: `codex-plugin-start-overlay-gate-20260223T044347Z-k3n8`
+
+- alias: `codex-plugin-start-overlay-gate`
+- mission: `Fix mpv plugin regression where start is refused if SubMiner process is not already running (TASK-117).`
+- status: `handoff`
+- branch: `main`
+- started_at: `2026-02-23T04:49:00Z`
+- heartbeat_minutes: `5`
+
+## Current Work (newest first)
+- [2026-02-23T04:47:40Z] completed: patched cold-start gate in `plugin/subminer.lua`, added Lua regression harness, validated red->green + syntax + focused launcher mpv tests.
+- [2026-02-23T04:49:00Z] intent: reproduce root cause from user logs; add red test first; patch plugin start gate with minimal behavior change.
+
+## Files Touched
+- `plugin/subminer.lua`
+- `scripts/test-plugin-start-gate.lua`
+- `backlog/tasks/task-117 - Fix-mpv-plugin-overlay-start-gate-when-SubMiner-is-not-running.md`
+- `docs/subagents/INDEX.md`
+- `docs/subagents/collaboration.md`
+- `docs/subagents/agents/codex-plugin-start-overlay-gate-20260223T044347Z-k3n8.md`
+
+## Assumptions
+- Regression introduced by `is_subminer_ipc_ready()` check at start of `start_overlay()`.
+- Cold-start should not require pre-existing process; socket checks remain valid once process exists.
+
+## Open Questions / Blockers
+- None.
+
+## Next Step
+- User runtime verification: launch with `SUBMINER_APPIMAGE_PATH=... ./dist/launcher/subminer --log-level debug ...` and trigger `y-s`.