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/done.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/git"
 	"github.com/steveyegge/gastown/internal/mail"
 	"github.com/steveyegge/gastown/internal/style"
@@ -213,8 +214,9 @@ func runDone(cmd *cobra.Command, args []string) error {
 		fmt.Printf("%s Witness notified of %s\n", style.Bold.Render("✓"), exitType)
 	}
 
-	// Log done event
+	// Log done event (townlog and activity feed)
 	LogDone(townRoot, sender, issueID)
+	_ = events.LogFeed(events.TypeDone, sender, events.DonePayload(issueID, branch))
 
 	// Update agent bead state (ZFC: self-report completion)
 	updateAgentStateOnDone(cwd, townRoot, exitType, issueID)