Instrument gt commands to appear in activity feed (gt-7aw1m)

Phase 1 of activity feed improvements: gt commands now log events to
~/gt/.events.jsonl. This is the raw audit log that the feed daemon
(phase 2) will curate into the user-facing feed.

Instrumented commands:
- gt sling: logs sling events with bead and target
- gt hook: logs hook events with bead
- gt handoff: logs handoff events with subject
- gt done: logs done events with bead and branch
- gt mail send: logs mail events with to and subject

Event format follows the specification:
```json
{"ts":"2025-12-30T07:36:28Z","source":"gt","type":"mail",
 "actor":"gastown/crew/joe","payload":{...},"visibility":"feed"}
```

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

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

internal/cmd/handoff.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/tmux"
@@ -127,13 +128,15 @@ func runHandoff(cmd *cobra.Command, args []string) error {
 	// Handing off ourselves - print feedback then respawn
 	fmt.Printf("%s Handing off %s...\n", style.Bold.Render("🤝"), currentSession)
 
-	// Log handoff event
+	// Log handoff event (both townlog and events feed)
 	if townRoot, err := workspace.FindFromCwd(); err == nil && townRoot != "" {
 		agent := sessionToGTRole(currentSession)
 		if agent == "" {
 			agent = currentSession
 		}
 		LogHandoff(townRoot, agent, handoffSubject)
+		// Also log to activity feed
+		_ = events.LogFeed(events.TypeHandoff, agent, events.HandoffPayload(handoffSubject, true))
 	}
 
 	// Dry run mode - show what would happen (BEFORE any side effects)