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/tests/test_plan_agent.py

@@ -0,0 +1,152 @@
+"""
+Tests for the Plan agent.
+
+Tests registration, API endpoints, and tool restrictions.
+"""
+import pytest
+
+from src.domains.agents.base import get_agent, list_agents
+from src.domains.agents.plan import plan_agent, PlanAgentImpl
+
+
+class TestPlanAgentRegistration:
+    """Tests for Plan agent registration."""
+
+    def test_plan_agent_registered(self):
+        """Test that plan agent is registered in registry."""
+        agent = get_agent("plan")
+        assert agent is not None
+        assert agent.name == "plan"
+
+    def test_plan_agent_in_list(self):
+        """Test that plan agent appears in agent list."""
+        agents = list_agents()
+        names = [a["name"] for a in agents]
+        assert "plan" in names
+
+    def test_plan_agent_has_description(self):
+        """Test that plan agent has a description."""
+        agent = get_agent("plan")
+        assert agent is not None
+        assert len(agent.description) > 0
+        assert "plan" in agent.description.lower() or "architect" in agent.description.lower()
+
+    def test_plan_agent_singleton(self):
+        """Test that plan_agent is the registered instance."""
+        registered = get_agent("plan")
+        assert registered is plan_agent
+
+    def test_plan_agent_is_correct_type(self):
+        """Test that plan agent is correct implementation type."""
+        assert isinstance(plan_agent, PlanAgentImpl)
+
+
+class TestPlanAgentTools:
+    """Tests for Plan agent tool restrictions."""
+
+    def test_plan_agent_has_read_only_tools(self):
+        """Test that plan agent has read-only tools."""
+        # Access the underlying PydanticAI agent to check tools
+        agent = plan_agent.agent
+        tool_names = list(agent._function_toolset.tools.keys())
+
+        # Should have read-only tools
+        assert "read_file" in tool_names
+        assert "glob_files" in tool_names
+        assert "grep_content" in tool_names
+        assert "bash_readonly" in tool_names
+
+    def test_plan_agent_no_write_tools(self):
+        """Test that plan agent does NOT have write tools."""
+        agent = plan_agent.agent
+        tool_names = list(agent._function_toolset.tools.keys())
+
+        # Should NOT have write tools
+        assert "edit_file" not in tool_names
+        assert "write_file" not in tool_names
+        assert "bash" not in tool_names
+        assert "web_search" not in tool_names
+
+    def test_plan_agent_tool_count(self):
+        """Test that plan agent has exactly 4 tools."""
+        agent = plan_agent.agent
+        tool_count = len(agent._function_toolset.tools)
+        assert tool_count == 4
+
+
+class TestPlanAgentAPI:
+    """Tests for Plan agent REST API."""
+
+    @pytest.mark.anyio
+    async def test_list_agents_includes_plan(self, auth_client):
+        """Test that agent list includes plan agent."""
+        response = await auth_client.get("/agents/")
+
+        assert response.status_code == 200
+        data = response.json()
+        names = [a["name"] for a in data["agents"]]
+        assert "plan" in names
+
+    @pytest.mark.anyio
+    async def test_get_plan_agent_info(self, auth_client):
+        """Test getting plan agent info."""
+        response = await auth_client.get("/agents/plan")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["name"] == "plan"
+        assert "description" in data
+        assert len(data["description"]) > 0
+
+    @pytest.mark.anyio
+    async def test_run_plan_with_invalid_body(self, auth_client):
+        """Test running plan agent with invalid request."""
+        response = await auth_client.post(
+            "/agents/run",
+            json={
+                "agent_type": "plan",
+                # Missing prompt
+            }
+        )
+
+        assert response.status_code == 422
+
+    @pytest.mark.anyio
+    async def test_stream_plan_with_invalid_body(self, auth_client):
+        """Test streaming plan agent with invalid request."""
+        response = await auth_client.post(
+            "/agents/stream",
+            json={
+                "agent_type": "plan",
+                # Missing prompt
+            }
+        )
+
+        assert response.status_code == 422
+
+
+class TestPlanAgentProperties:
+    """Tests for Plan agent properties and configuration."""
+
+    def test_plan_agent_name(self):
+        """Test plan agent name property."""
+        assert plan_agent.name == "plan"
+
+    def test_plan_agent_description_not_empty(self):
+        """Test plan agent description is not empty."""
+        assert plan_agent.description
+        assert len(plan_agent.description) > 10
+
+    def test_plan_agent_creates_agent_lazily(self):
+        """Test that PydanticAI agent is created lazily."""
+        # Create a fresh instance
+        fresh_agent = PlanAgentImpl()
+
+        # _agent should be None before first access
+        assert fresh_agent._agent is None
+
+        # Access the agent property
+        _ = fresh_agent.agent
+
+        # Now _agent should be set
+        assert fresh_agent._agent is not None