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/spawn.go

@@ -461,6 +461,9 @@ func runSpawn(cmd *cobra.Command, args []string) error {
 		style.Bold.Render("✓"),
 		style.Dim.Render(fmt.Sprintf("gt session at %s/%s", rigName, polecatName)))
 
+	// Log spawn event
+	LogSpawn(townRoot, polecatAddress, assignmentID)
+
 	// NOTE: We do NOT send a nudge here. Claude Code takes 10-20+ seconds to initialize,
 	// and sending keys before the prompt is ready causes them to be mangled.
 	// The Deacon will poll with WaitForClaudeReady and send a trigger when ready.