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/manager_test.go

@@ -9,21 +9,22 @@ import (
 	"github.com/steveyegge/gastown/internal/rig"
 )
 
-func TestStateIsAvailable(t *testing.T) {
+func TestStateIsActive(t *testing.T) {
 	tests := []struct {
-		state     State
-		available bool
+		state  State
+		active bool
 	}{
-		{StateIdle, true},
-		{StateActive, true},
-		{StateWorking, false},
+		{StateWorking, true},
 		{StateDone, false},
 		{StateStuck, false},
+		// Legacy states are treated as active
+		{StateIdle, true},
+		{StateActive, true},
 	}
 
 	for _, tt := range tests {
-		if got := tt.state.IsAvailable(); got != tt.available {
-			t.Errorf("%s.IsAvailable() = %v, want %v", tt.state, got, tt.available)
+		if got := tt.state.IsActive(); got != tt.active {
+			t.Errorf("%s.IsActive() = %v, want %v", tt.state, got, tt.active)
 		}
 	}
 }
@@ -299,7 +300,7 @@ func TestClearIssue(t *testing.T) {
 		t.Fatalf("ClearIssue: %v", err)
 	}
 
-	// Verify
+	// Verify - in ephemeral model, ClearIssue transitions to Done
 	polecat, err := m.Get("Test")
 	if err != nil {
 		t.Fatalf("Get: %v", err)
@@ -307,7 +308,7 @@ func TestClearIssue(t *testing.T) {
 	if polecat.Issue != "" {
 		t.Errorf("Issue = %q, want empty", polecat.Issue)
 	}
-	if polecat.State != StateIdle {
-		t.Errorf("State = %v, want StateIdle", polecat.State)
+	if polecat.State != StateDone {
+		t.Errorf("State = %v, want StateDone", polecat.State)
 	}
 }