feat: implement ephemeral polecat model

This implements the ephemeral polecat model where polecats are spawned
fresh for each task and deleted upon completion.

Key changes:

**Spawn (internal/cmd/spawn.go):**
- Always create fresh worktree from main branch
- Run bd init in new worktree to initialize beads
- Remove --create flag (now implicit)
- Replace stale polecats with fresh worktrees

**Handoff (internal/cmd/handoff.go):**
- Add rig/polecat detection from environment and tmux session
- Send shutdown requests to correct witness (rig/witness)
- Include polecat name in lifecycle request body

**Witness (internal/witness/manager.go):**
- Add mail checking in monitoring loop
- Process LIFECYCLE shutdown requests
- Implement full cleanup sequence:
  - Kill tmux session
  - Remove git worktree
  - Delete polecat branch

**Polecat state machine (internal/polecat/types.go):**
- Primary states: working, done, stuck
- Deprecate idle/active (kept for backward compatibility)
- New polecats start in working state
- ClearIssue transitions to done (not idle)

**Polecat commands (internal/cmd/polecat.go):**
- Update list to show "Active Polecats"
- Normalize legacy states for display
- Add deprecation warnings to wake/sleep commands

Closes gt-7ik

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

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

internal/polecat/types.go

@@ -4,35 +4,39 @@ package polecat
 import "time"
 
 // State represents the current state of a polecat.
+// In the ephemeral model, polecats exist only while working.
 type State string
 
 const (
-	// StateIdle means the polecat is not actively working.
-	StateIdle State = "idle"
-
-	// StateActive means the polecat session is running but not assigned work.
-	StateActive State = "active"
-
 	// StateWorking means the polecat is actively working on an issue.
+	// This is the initial and primary state for ephemeral polecats.
 	StateWorking State = "working"
 
-	// StateDone means the polecat has completed its assigned work.
+	// StateDone means the polecat has completed its assigned work
+	// and is ready for cleanup by the Witness.
 	StateDone State = "done"
 
 	// StateStuck means the polecat needs assistance.
 	StateStuck State = "stuck"
-)
 
-// IsAvailable returns true if the polecat can be assigned new work.
-func (s State) IsAvailable() bool {
-	return s == StateIdle || s == StateActive
-}
+	// Legacy states for backward compatibility during transition.
+	// New code should not use these.
+	StateIdle   State = "idle"   // Deprecated: use StateWorking
+	StateActive State = "active" // Deprecated: use StateWorking
+)
 
 // IsWorking returns true if the polecat is currently working.
 func (s State) IsWorking() bool {
 	return s == StateWorking
 }
 
+// IsActive returns true if the polecat session is actively working.
+// For ephemeral polecats, this is true for working state and
+// legacy idle/active states (treated as working).
+func (s State) IsActive() bool {
+	return s == StateWorking || s == StateIdle || s == StateActive
+}
+
 // Polecat represents a worker agent in a rig.
 type Polecat struct {
 	// Name is the polecat identifier.