Closes GH#516: bd init now automatically:
- Creates AGENTS.md and @AGENTS.md with landing-the-plane instructions if they don't exist
- Appends the instructions to existing files if they don't have them
- Skips if the section already exists (idempotent)
- Skips in stealth mode (user wants invisible setup)
The landing-the-plane instructions ensure AI agents properly complete
their work sessions by pushing all changes to remote before ending.
8e1a841daa