MAESTRO: Implement Phase 8.2 - Update documentation for multi-provider architecture

- CLAUDE.md: Added agent capabilities section, updated architecture diagram
  with parsers/ and storage/ directories, added agentSessions and agentError
  APIs, updated key files table, added error fields to Session interface
- AGENT_SUPPORT.md: Added multi-provider architecture status section showing
  all 7 completed components, updated supported agents reference with
  implementation status and detailed checklist for planned agents
- CONTRIBUTING.md: Updated supported agents reference table with status
  column and link to detailed guide

CONTRIBUTING.md

@@ -425,12 +425,14 @@ Based on capabilities, these UI features are automatically enabled/disabled:
 
 ### Supported Agents Reference
 
-| Agent | Resume | Read-Only | JSON | Images | Sessions | Cost |
-|-------|--------|-----------|------|--------|----------|------|
-| Claude Code | ✅ `--resume` | ✅ `--permission-mode plan` | ✅ | ✅ | ✅ `~/.claude/` | ✅ |
-| OpenCode | ✅ `--session` | ✅ `--agent plan` | ✅ | ✅ | TBD | ❌ (local) |
-| Gemini CLI | TBD | TBD | TBD | TBD | TBD | ✅ |
-| Codex | TBD | TBD | TBD | TBD | TBD | ✅ |
+| Agent | Resume | Read-Only | JSON | Images | Sessions | Cost | Status |
+|-------|--------|-----------|------|--------|----------|------|--------|
+| Claude Code | ✅ `--resume` | ✅ `--permission-mode plan` | ✅ | ✅ | ✅ `~/.claude/` | ✅ | ✅ Complete |
+| OpenCode | ✅ `--session` | ✅ `--agent plan` | ✅ | ✅ | Stub | ❌ (local) | 🔄 Stub Ready |
+| Gemini CLI | TBD | TBD | TBD | TBD | TBD | ✅ | 📋 Planned |
+| Codex | TBD | TBD | TBD | TBD | TBD | ✅ | 📋 Planned |
+
+For detailed implementation guide, see [AGENT_SUPPORT.md](AGENT_SUPPORT.md).
 
 ## Code Style