feat: agent coordination system (workflows, locks, components, messages)

Adds /api/coord/* endpoints for real-time cross-session coordination:
- coord_workflows: named units of work per project
- coord_work_items: tasks within workflows with dependency chains
- coord_session_locks: exclusive resource locks with auto-expiry (TTL)
- coord_component_states: live component state per project (upsert)
- coord_messages: cross-session messaging and broadcasts
- /api/coord/status: cross-project snapshot endpoint

Replaces PROJECT_STATE.md as the coordination layer for Claude sessions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/schemas/coord_message.py

@@ -0,0 +1,39 @@
+"""Pydantic schemas for CoordMessage."""
+
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+class CoordMessageCreate(BaseModel):
+    """Input schema for sending a message."""
+
+    from_session: str = Field(..., description="Sending session identifier", max_length=200)
+    to_session: Optional[str] = Field(None, description="Recipient session; NULL = broadcast", max_length=200)
+    project_key: Optional[str] = Field(None, description="Optional project context", max_length=200)
+    subject: str = Field(..., description="Message subject", max_length=500)
+    body: str = Field(..., description="Message body, markdown ok")
+
+
+class CoordMessageUpdate(BaseModel):
+    """Input schema for updating a message (mark read, etc.)."""
+
+    read_at: Optional[datetime] = None
+
+
+class CoordMessageResponse(BaseModel):
+    """Output schema for a message."""
+
+    id: UUID
+    from_session: str
+    to_session: Optional[str]
+    project_key: Optional[str]
+    subject: str
+    body: str
+    read_at: Optional[datetime]
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}