feat(CUST-WP-0015): implement agent inbox for inter-agent coordination

Adds a message-passing layer to state-hub so Claude instances can
coordinate across sessions without polling shared progress events.

- Migration f3a4b5c6d7e8: agent_messages table with thread support
- FastAPI router: POST/GET /messages/, thread view, mark-read, archive, reply
- 4 MCP tools: send_message, get_messages, mark_message_read, reply_to_message
- Observable dashboard: /inbox page with unread/read/archived sections + KPI
- CLAUDE.md updates: global, custodian, marki-docx, activity-core, template
- TOOLS.md: Agent Inbox tools section documented

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

api/models/__init__.py

@@ -14,6 +14,7 @@ from api.models.technical_debt import TechnicalDebt, TDStatus
 from api.models.contribution import Contribution, ContributionType, ContributionStatus
 from api.models.sbom_snapshot import SBOMSnapshot
 from api.models.sbom_entry import SBOMEntry, Ecosystem
+from api.models.agent_message import AgentMessage
 
 __all__ = [
     "Base",
@@ -32,4 +33,5 @@ __all__ = [
     "Contribution", "ContributionType", "ContributionStatus",
     "SBOMSnapshot",
     "SBOMEntry", "Ecosystem",
+    "AgentMessage",
 ]

api/models/agent_message.py

@@ -0,0 +1,44 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, Text, text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from api.models.base import Base, new_uuid
+
+
+class AgentMessage(Base):
+    __tablename__ = "agent_messages"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=new_uuid
+    )
+    from_agent: Mapped[str] = mapped_column(String(100), nullable=False)
+    to_agent: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    subject: Mapped[str] = mapped_column(String(500), nullable=False)
+    body: Mapped[str] = mapped_column(Text, nullable=False)
+    thread_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("agent_messages.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    read_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    archived_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=text("now()"),
+        nullable=False,
+    )
+
+    thread_root: Mapped["AgentMessage | None"] = relationship(
+        "AgentMessage",
+        remote_side="AgentMessage.id",
+        foreign_keys=[thread_id],
+        lazy="select",
+    )