* feat(opencode): add OpenCode plugin support
Add `--opencode` flag to `rtk init` for installing a global OpenCode
plugin that rewrites Bash/shell commands through `rtk rewrite`.
- New plugin: hooks/opencode-rtk.ts (thin delegator to rtk rewrite)
- New init modes: --opencode (OpenCode only), combinable with Claude modes
- Plugin install/update/remove lifecycle with idempotent writes
- Uninstall cleans up OpenCode plugin alongside Claude Code artifacts
- `rtk init --show` reports OpenCode plugin status
- Replace unreachable!() with bail!() in match exhaustiveness guard
* docs: add OpenCode plugin documentation
- README: OpenCode plugin section, install flags, troubleshooting
- TROUBLESHOOTING: OpenCode-specific checklist
- Update init mode table to reflect Claude Code default
José Almeida