feat(crew): Provision .claude/commands/ for crew and polecat workspaces (gt-jhr85)

When adding a crew member with 'gt crew add' or spawning a polecat,
provision the .claude/commands/ directory with standard slash commands
like /handoff. This ensures all agents have Gas Town utilities even if
the source repo does not have them tracked.

Changes:
- Add embedded commands templates (internal/templates/commands/)
- Add ProvisionCommands() to templates package
- Call ProvisionCommands from crew and polecat managers
- Add gt doctor commands-provisioned check with --fix support

internal/templates/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., gastown/crew/max for crew, mayor/ for mayor).
+   Example: `gt mail send gastown/crew/max -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.