feat: add Plan Agent for implementation planning

- Add PlanAgentImpl with read-only tools only
- System prompts optimized for architecture planning
- Outputs step-by-step implementation plans with critical files
- 15 unit tests for registration, tools, and API
- Update COVERAGE.md to ~70% complete

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/domains/agents/router.py

@@ -9,6 +9,7 @@ from src.domains.agents.base import get_agent, list_agents
 
 # Import agents to ensure they're registered
 import src.domains.agents.explore  # noqa: F401
+import src.domains.agents.plan  # noqa: F401
 from src.domains.agents.schemas import (
     AgentRunRequest,
     AgentRunResponse,