feat: preflight git-gate host key population

When a git-gate repo entry has no host_key configured, the prepare step
now either errors (headless) or fetches the key via ssh-keyscan, shows
it to the operator for confirmation, and optionally persists it to the
bottle config .md file on disk.

Closes #333

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/cli/start.py

@@ -209,6 +209,7 @@ def _start_headless(
         label=label,
         color=args.color or "",
         bottle_names=bottle_names,
+        headless=True,
     )
     return _launch_bottle(
         spec,