Detect and clean stale POLECAT_DONE messages (#913)

* fix(witness): detect and ignore stale POLECAT_DONE messages

Add timestamp validation to prevent witness from nuking newly spawned
polecat sessions when processing stale POLECAT_DONE messages from
previous sessions.

- Add isStalePolecatDone() to compare message timestamp vs session created time
- If message timestamp < session created time, message is stale and ignored
- Add unit tests for timestamp parsing and stale detection logic

Fixes #909

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

* feat(mail): add --stale flag to gt mail archive

Add ability to archive stale messages (sent before current session started).
This prevents old messages from cycling forever in patrol inbox.

Changes:
- Add --stale and --dry-run flags to gt mail archive
- Move stale detection helpers to internal/session/stale.go for reuse
- Add ParseAddress to parse mail addresses into AgentIdentity
- Add SessionCreatedAt to get tmux session start time

Usage:
  gt mail archive --stale           # Archive all stale messages
  gt mail archive --stale --dry-run # Preview what would be archived

Co-Authored-By: GPT-5.2 Codex <noreply@openai.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: GPT-5.2 Codex <noreply@openai.com>

internal/session/identity_test.go

@@ -250,3 +250,71 @@ func TestParseSessionName_RoundTrip(t *testing.T) {
 		})
 	}
 }
+
+func TestParseAddress(t *testing.T) {
+	tests := []struct {
+		name    string
+		address string
+		want    AgentIdentity
+		wantErr bool
+	}{
+		{
+			name:    "mayor",
+			address: "mayor/",
+			want:    AgentIdentity{Role: RoleMayor},
+		},
+		{
+			name:    "deacon",
+			address: "deacon",
+			want:    AgentIdentity{Role: RoleDeacon},
+		},
+		{
+			name:    "witness",
+			address: "gastown/witness",
+			want:    AgentIdentity{Role: RoleWitness, Rig: "gastown"},
+		},
+		{
+			name:    "refinery",
+			address: "rig-a/refinery",
+			want:    AgentIdentity{Role: RoleRefinery, Rig: "rig-a"},
+		},
+		{
+			name:    "crew",
+			address: "gastown/crew/max",
+			want:    AgentIdentity{Role: RoleCrew, Rig: "gastown", Name: "max"},
+		},
+		{
+			name:    "polecat explicit",
+			address: "gastown/polecats/nux",
+			want:    AgentIdentity{Role: RolePolecat, Rig: "gastown", Name: "nux"},
+		},
+		{
+			name:    "polecat canonical",
+			address: "gastown/nux",
+			want:    AgentIdentity{Role: RolePolecat, Rig: "gastown", Name: "nux"},
+		},
+		{
+			name:    "invalid",
+			address: "gastown/crew",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ParseAddress(tt.address)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("ParseAddress(%q) error = %v", tt.address, err)
+			}
+			if *got != tt.want {
+				t.Fatalf("ParseAddress(%q) = %#v, want %#v", tt.address, *got, tt.want)
+			}
+		})
+	}
+}