docs: update AGENTS.md hierarchy with openclaw, runtime-fallback, skill-mcp-manager

- Add src/openclaw/AGENTS.md: bidirectional Discord/Telegram/webhook integration
- Add src/hooks/runtime-fallback/AGENTS.md: reactive provider error recovery
- Add src/features/skill-mcp-manager/AGENTS.md: tier-3 MCP lifecycle
- Update root AGENTS.md: refresh commit hash, add openclaw/IntentGate/Hashline refs
- Fix src/features/AGENTS.md: skill-mcp-manager file count 14→18, complexity MEDIUM→HIGH

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>