Implement town activity logging (gt-ewzon)

Add centralized logging for Gas Town agent lifecycle events:
- spawn: new agent created
- wake: agent resumed
- nudge: message injected
- handoff: agent handed off
- done: agent finished work
- crash: agent exited unexpectedly
- kill: agent killed intentionally

Implementation:
- Add internal/townlog package with LogEvent() function
- Log to ~/gt/logs/town.log with human-readable format
- Add gt log command to view/tail/filter logs
- Integrate logging into spawn, nudge, handoff, done, stop, session commands

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

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

internal/cmd/done.go

@@ -220,5 +220,8 @@ func runDone(cmd *cobra.Command, args []string) error {
 		fmt.Printf("%s Witness notified of %s\n", style.Bold.Render("✓"), exitType)
 	}
 
+	// Log done event
+	LogDone(townRoot, sender, issueID)
+
 	return nil
 }