feat: Show actor on pinned/status change events (gt-1ydd9)

- Add Actor field to MutationEvent struct
- Use new assignee from update args instead of old issue state
- Include actor (who performed the action) in mutation events
- Display actor in bd activity output, falling back to assignee

When pinning/updating status, the activity feed now shows who performed
the action (e.g., "@gastown/crew/jack") instead of showing nothing.

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

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

internal/rpc/server_core.go

@@ -84,6 +84,7 @@ type MutationEvent struct {
 	IssueID   string    // e.g., "bd-42"
 	Title     string    // Issue title for display context (may be empty for some operations)
 	Assignee  string    // Issue assignee for display context (may be empty)
+	Actor     string    // Who performed the action (may differ from assignee)
 	Timestamp time.Time
 	// Optional metadata for richer events (used by status, bonded, etc.)
 	OldStatus string `json:"old_status,omitempty"` // Previous status (for status events)