feat: add /handoff slash command for user invocation (bd-xwvo)

Beads only had the skill version (Claude-only). This adds the command
version so users can type /handoff directly like in gastown.

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

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

.claude/commands/handoff.md

@@ -0,0 +1,21 @@
+---
+description: Hand off to fresh session, work continues from hook
+allowed-tools: Bash(gt mail send:*),Bash(gt handoff:*)
+argument-hint: [message]
+---
+
+Hand off to a fresh session.
+
+User's handoff message (if any): $ARGUMENTS
+
+Execute these steps in order:
+
+1. If user provided a message, send handoff mail to yourself first.
+   Construct your mail address from your identity (e.g., beads/crew/emma for crew).
+   Example: `gt mail send beads/crew/emma -s "HANDOFF: Session cycling" -m "USER_MESSAGE_HERE"`
+
+2. Run the handoff command (this will respawn your session with a fresh Claude):
+   `gt handoff`
+
+Note: The new session will auto-prime via the SessionStart hook and find your handoff mail.
+End watch. A new session takes over, picking up any molecule on the hook.