docs: document polecat lifecycle (session/sandbox/slot layers)

Clarifies the three distinct lifecycle layers to prevent confusion:
- Session (Claude context): ephemeral, cycles per step
- Sandbox (worktree): persistent until nuke
- Slot (name from pool): persistent until nuke

Addresses anti-patterns like "idle polecats" and misunderstanding
what recycling means. (gt-bc6gm)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/polecat-CLAUDE.md

@@ -182,6 +182,9 @@ to the merge queue. Without this step:
 
 ## Self-Managed Session Lifecycle
 
+> See [Polecat Lifecycle](docs/polecat-lifecycle.md) for the full three-layer architecture
+> (session/sandbox/slot).
+
 **You own your session cadence.** The Witness monitors but doesn't force recycles.
 
 ### Closing Steps (for Activity Feed)