Add OpenClaw as a fourth supported agent runtime alongside Claude Code,
Codex, and OpenCode. OpenClaw CLI (`openclaw agent -p ... --output-format
stream-json`) is integrated via the same Backend interface pattern.
Changes:
- Add openclawBackend in server/pkg/agent/openclaw.go with NDJSON
event stream parsing (text, thinking, tool_call, error, step, result)
- Register "openclaw" in the agent factory (agent.go)
- Add MULTICA_OPENCLAW_PATH / MULTICA_OPENCLAW_MODEL env var detection
in daemon config
- Include "openclaw" in AGENTS.md config injection alongside codex/opencode
- Add comprehensive unit tests for all event handlers and processEvents