docs: port full G-commit section from forest AGENTS.md [AGENT]
Expanded G-commit golden rule to match forest's version:
- Added explicit instruction to commit before pausing for further instructions
- Added constraint against committing unrelated files from working directory
Replaced minimal commit discipline section with full jj workflow from forest:
- Pre-commit: use `jj` status/log, `jj diff` to review changes
- During commit: granular/targeted commits with jj path syntax, no sensitive info
- Post-commit: how to edit messages with `jj desc`
- Full commit message checklist: conventional commits, descriptive title+body, [AGENT] tag
- Added human discipline note about reviewing AI-generated code