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/models/coord_message.py

@@ -0,0 +1,56 @@
+"""Coordination inter-session message model."""
+
+from datetime import datetime
+from typing import Optional
+
+from sqlalchemy import DateTime, Index, String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class CoordMessage(Base, UUIDMixin, TimestampMixin):
+    """A message sent from one session to another (or broadcast)."""
+
+    __tablename__ = "coord_messages"
+
+    from_session: Mapped[str] = mapped_column(
+        String(200),
+        nullable=False,
+        doc="Sending session, e.g. 'DESKTOP-0O8A1RL/Claude'"
+    )
+
+    to_session: Mapped[Optional[str]] = mapped_column(
+        String(200),
+        doc="Recipient session; NULL means broadcast to all"
+    )
+
+    project_key: Mapped[Optional[str]] = mapped_column(
+        String(200),
+        doc="Optional project context for the message"
+    )
+
+    subject: Mapped[str] = mapped_column(
+        String(500),
+        nullable=False,
+        doc="Message subject line"
+    )
+
+    body: Mapped[str] = mapped_column(
+        Text,
+        nullable=False,
+        doc="Message body, markdown ok"
+    )
+
+    read_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime,
+        doc="NULL means unread"
+    )
+
+    __table_args__ = (
+        Index("idx_coord_messages_to_read", "to_session", "read_at"),
+        Index("idx_coord_messages_from", "from_session"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<CoordMessage(from='{self.from_session}', to='{self.to_session}', subject='{self.subject}')>"