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/sling.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/gastown/internal/beads"
+	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/tmux"
@@ -330,6 +331,10 @@ func runSling(cmd *cobra.Command, args []string) error {
 
 	fmt.Printf("%s Work attached to hook (status=hooked)\n", style.Bold.Render("✓"))
 
+	// Log sling event to activity feed
+	actor := detectActor()
+	_ = events.LogFeed(events.TypeSling, actor, events.SlingPayload(beadID, targetAgent))
+
 	// Update agent bead's hook_bead field (ZFC: agents track their current work)
 	updateAgentHookBead(targetAgent, beadID)
 
@@ -688,6 +693,12 @@ func runSlingFormula(args []string) error {
 	}
 	fmt.Printf("%s Attached to hook (status=hooked)\n", style.Bold.Render("✓"))
 
+	// Log sling event to activity feed (formula slinging)
+	actor := detectActor()
+	payload := events.SlingPayload(wispResult.RootID, targetAgent)
+	payload["formula"] = formulaName
+	_ = events.LogFeed(events.TypeSling, actor, payload)
+
 	// Update agent bead's hook_bead field (ZFC: agents track their current work)
 	updateAgentHookBead(targetAgent, wispResult.RootID)
 
@@ -768,6 +779,15 @@ func wakeRigAgents(rigName string) {
 	_ = t.NudgeSession(refinerySession, "Polecat dispatched - check for merge requests")
 }
 
+// detectActor returns the current agent's actor string for event logging.
+func detectActor() string {
+	roleInfo, err := GetRole()
+	if err != nil {
+		return "unknown"
+	}
+	return roleInfo.ActorString()
+}
+
 // agentIDToBeadID converts an agent ID to its corresponding agent bead ID.
 // Uses canonical naming: prefix-rig-role-name
 func agentIDToBeadID(agentID string) string {