Add RTK support for OpenCode alongside existing Claude Code integration.
OpenCode users can now install RTK via its native plugin system for
transparent command rewriting and token optimization.

New files:
- hooks/opencode-rtk-plugin.ts: TypeScript plugin that ports the bash
  rewrite logic to OpenCode's tool.execute.before hook
- hooks/opencode-rtk-awareness.md: Slim awareness doc for OpenCode
- hooks/opencode-rtk-rules.md: Rules file for OpenCode's rules system

CLI changes:
- rtk init --opencode [-g]: Install plugin + rules for OpenCode
- rtk init --opencode --show: Show OpenCode RTK configuration
- rtk init --opencode --uninstall [-g]: Remove OpenCode RTK artifacts
- rtk discover --opencode: Scan OpenCode sessions for missed savings

Implementation:
- OpenCode plugin uses tool.execute.before event to intercept and
  rewrite bash commands (same patterns as rtk-rewrite.sh)
- OpenCodeProvider in discover module supports session scanning
- All 418 tests pass, no new clippy errors