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.go

@@ -42,6 +42,10 @@ var (
 
 	// Clear flags
 	mailClearAll bool
+
+	// Archive flags
+	mailArchiveStale  bool
+	mailArchiveDryRun bool
 )
 
 var mailCmd = &cobra.Command{
@@ -196,16 +200,22 @@ Examples:
 }
 
 var mailArchiveCmd = &cobra.Command{
-	Use:   "archive <message-id> [message-id...]",
+	Use:   "archive [message-id...]",
 	Short: "Archive messages",
 	Long: `Archive one or more messages.
 
 Removes the messages from your inbox by closing them in beads.
 
+Use --stale to archive messages sent before your current session started.
+
 Examples:
-  gt mail archive hq-abc123
-  gt mail archive hq-abc123 hq-def456 hq-ghi789`,
-	Args: cobra.MinimumNArgs(1),
+	gt mail archive hq-abc123
+	gt mail archive hq-abc123 hq-def456 hq-ghi789
+	gt mail archive --stale
+	gt mail archive --stale --dry-run`,
+	Args: func(cmd *cobra.Command, args []string) error {
+		return nil
+	},
 	RunE: runMailArchive,
 }
 
@@ -487,6 +497,10 @@ func init() {
 	// Clear flags
 	mailClearCmd.Flags().BoolVar(&mailClearAll, "all", false, "Clear all messages (default behavior)")
 
+	// Archive flags
+	mailArchiveCmd.Flags().BoolVar(&mailArchiveStale, "stale", false, "Archive messages sent before session start")
+	mailArchiveCmd.Flags().BoolVarP(&mailArchiveDryRun, "dry-run", "n", false, "Show what would be archived without archiving")
+
 	// Add subcommands
 	mailCmd.AddCommand(mailSendCmd)
 	mailCmd.AddCommand(mailInboxCmd)