Remove StateIdle and idle polecat concept

The transient polecat model says: "Polecats exist only while working."
This removes the deprecated StateIdle and updates the codebase:

- Remove StateIdle from polecat/types.go (keep StateActive for legacy data)
- Update manager.go: Get() returns StateDone (not StateIdle) when no work
- Update manager.go: Add/Recreate return StateWorking (not StateIdle)
- Remove zombie scan logic from deacon.go (no idle polecats to scan for)
- Update tests to reflect new behavior

The correct lifecycle is now:
- Spawn: polecat created with work (StateWorking)
- Work: sessions cycle, sandbox persists
- Done: polecat signals completion (StateDone)
- Nuke: Witness destroys sandbox

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

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

internal/polecat/types.go

@@ -19,10 +19,9 @@ const (
 	// StateStuck means the polecat needs assistance.
 	StateStuck State = "stuck"
 
-	// 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
+	// StateActive is deprecated: use StateWorking.
+	// Kept only for backward compatibility with existing data.
+	StateActive State = "active"
 )
 
 // IsWorking returns true if the polecat is currently working.
@@ -32,9 +31,9 @@ func (s State) IsWorking() bool {
 
 // IsActive returns true if the polecat session is actively working.
 // For transient polecats, this is true for working state and
-// legacy idle/active states (treated as working).
+// legacy active state (treated as working).
 func (s State) IsActive() bool {
-	return s == StateWorking || s == StateIdle || s == StateActive
+	return s == StateWorking || s == StateActive
 }
 
 // Polecat represents a worker agent in a rig.