**OpenCode Integration:**
- Add platform abstraction layer (src/platform.rs) for Claude Code and OpenCode
- New OpenCode plugin (hooks/rtk-rewrite.ts) with full command rewrite logic
- OpenCode session provider for discover/learn commands (scans ~/.local/share/opencode/opencode.db)
- Multi-platform support: discover/learn scan both platforms by default
- Updated documentation (README, INSTALL, ARCHITECTURE, TROUBLESHOOTING) for dual-platform usage

**Hook Fixes:**
- Fix critical basename detection bug: pattern `*/rtk\ *` incorrectly matched `git -C /path/to/rtk status`
- Changed to basename checking: only skip if first command word is literally 'rtk'
- Add workaround for git -C/-c flags (not yet supported by RTK core)
  - Hooks now skip rewriting git commands with global options
  - Prevents "unexpected argument '-C' found" errors
  - Documented as known limitation in TROUBLESHOOTING.md

**Testing:**
- All 435 tests pass
- Manual testing confirms hooks work in both platforms
- Hook correctly rewrites: `git status` → `rtk git status`
- Hook correctly skips: `git -C /path status` (no rewrite)
- Hook correctly skips: `rtk git status` (already using rtk)

**Files Changed:**
- NEW: hooks/rtk-rewrite.ts (OpenCode plugin)
- NEW: src/platform.rs (platform abstraction)
- FIXED: hooks/rtk-rewrite.sh (basename detection + git -C skip)
- UPDATED: src/discover/, src/init.rs, src/learn/ (multi-platform)
- DOCS: README, INSTALL, ARCHITECTURE, TROUBLESHOOTING