rules
- Introduce `rules` option to provide global custom instructions for opencode - Write `rules` content to `~/.config/opencode/AGENTS.md` if non-empty - Update tests to cover presence and absence of `AGENTS.md` file with rules content
We don't need to maintain them in here, removing them to fix CI check Signed-off-by: Austin Horstman <khaneliman12@gmail.com>