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/hook.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/gastown/internal/beads"
+	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/style"
 )
 
@@ -164,6 +165,9 @@ func runHook(cmd *cobra.Command, args []string) error {
 	fmt.Printf("  Use 'gt handoff' to restart with this work\n")
 	fmt.Printf("  Use 'gt mol status' to see hook status\n")
 
+	// Log hook event to activity feed
+	_ = events.LogFeed(events.TypeHook, agentID, events.HookPayload(beadID))
+
 	return nil
 }