Rename gt recycle to gt handoff

Consistent naming throughout:
- internal/cmd/recycle.go → handoff.go
- All variable/function names updated
- /handoff Claude Code command updated
- polecat.md prompt updated

Also includes session.go doc improvements (nudge preference).

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

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

.claude/commands/handoff.md

@@ -1,12 +1,12 @@
 ---
 description: Hand off to fresh session, work continues from hook
-allowed-tools: Bash(gt recycle)
+allowed-tools: Bash(gt handoff)
 ---
 
-Execute `gt recycle` to hand off to a fresh session:
+Execute `gt handoff` to hand off to a fresh session:
 
 ```bash
-gt recycle
+gt handoff
 ```
 
 End watch. A new session takes over, picking up any molecule on the hook.