feat: implement all adapter layers (#2)

Adapters implemented:
- adapters/llm/anthropic.py — Anthropic Claude SDK, capability-based model selection,
  max_tokens + temperature configurable via team.yaml, lazy SDK import
- adapters/vcs/github.py — GitHub PR/branch operations via gh CLI
- adapters/notify/openclaw.py — OpenClaw system event notifications
- adapters/runtime/openclaw.py — OpenClaw sessions_spawn for agent execution
- adapters/runtime/claude_code.py — Claude Code CLI for T4/T5 coding tasks

All adapters follow the abstract base interfaces from Phase 1.
Config-driven model selection via capability_map in team.yaml.

requirements.txt

@@ -10,6 +10,9 @@ pyyaml
 # Environment variable management
 python-dotenv
 
+# GitHub VCS adapter
+PyGithub
+
 # --- stdlib-only (no pip install needed) ---
 # sqlite3   — blackboard persistence
 # dataclasses — task_brief schema