Include various tests for the agents and command file generation and
different ways of configuring it.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
- 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