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/cmd/mail_archive_test.go

@@ -0,0 +1,25 @@
+package cmd
+
+import (
+	"testing"
+	"time"
+
+	"github.com/steveyegge/gastown/internal/mail"
+)
+
+func TestStaleMessagesForSession(t *testing.T) {
+	sessionStart := time.Date(2026, 1, 24, 2, 0, 0, 0, time.UTC)
+	messages := []*mail.Message{
+		{ID: "msg-1", Subject: "Older", Timestamp: sessionStart.Add(-2 * time.Minute)},
+		{ID: "msg-2", Subject: "Newer", Timestamp: sessionStart.Add(2 * time.Minute)},
+		{ID: "msg-3", Subject: "Equal", Timestamp: sessionStart},
+	}
+
+	stale := staleMessagesForSession(messages, sessionStart)
+	if len(stale) != 1 {
+		t.Fatalf("expected 1 stale message, got %d", len(stale))
+	}
+	if stale[0].Message.ID != "msg-1" {
+		t.Fatalf("expected msg-1 stale, got %s", stale[0].Message.ID)
+	}
+}