docs(lanes): document ccam lanes gc (E, F3c)

docs/LANES.md

@@ -810,6 +810,21 @@ Set the environment variable to change when a watcher is considered dead:
 export LANE_DEAD_SEC=600  # 10 minutes instead of 5
 ```
 
+## Housekeeping: ccam lanes gc
+
+Runs machine-wide, not against one lane — safe to run any time, running lanes and live sessions are unaffected:
+
+```bash
+ccam lanes gc [--dry-run]
+```
+
+Two things, both scoped to what's actually stale:
+
+- **Reaps orphaned Playwright MCP processes.** A `.playwright-mcp`-scoped process whose owning Claude Code session died gets reparented to pid 1 — a live session's MCP keeps its real parent and is left alone. Kills the whole process tree so a spawned browser doesn't leak too.
+- **Caps oversized hook logs.** Any `LANES_ROOT/.state/lane<N>/logs/*.log` over 10MB is truncated in place to its last 2MB (same inode — a concurrent writer's file descriptor stays valid).
+
+`--dry-run` prints what would happen without doing it. This is a deliberately narrower port of Shipyard's `lane-gc.sh` — it does NOT auto-remove stale worktrees by age (that would violate this repo's own never-automatic-destroy rule; use `ccam lanes reset|remove|purge` explicitly instead), archive feature state (CCAM's lane/feature state lives in the database, not flat files), or sweep scratch debris (CCAM doesn't generate the files Shipyard's own scripts did). See `docs/superpowers/specs/2026-08-05-lane-gc-design.md` for the full scoping rationale.
+
 ## Lane actions
 
 The web UI and CLI provide these actions on a lane: