STATE-WP-0093: per-recipient broadcast receipts and standing notices (T01-T06).

Founder approved T01 on 2026-09-22 (D2, D3, D6 as recommended).
- T02: message_receipts table; kind/expires_at/supersedes_id on
  agent_messages; migration d7e8f9a0b1c2 archives existing broadcasts,
  leaves direct messages untouched, reversible.
- T03: reader-aware mark-read (unattributed broadcast mark-read is a
  metered, deprecated no-op), delivery receipts on the scoped unread inbox,
  POST /messages/{id}/ack, news/standing kinds, expiry, supersede, broadcast
  archive no longer stamps read_at, reply writes the replier's receipt.
- T04 (state-hub part): Codex MCP reader param and acknowledge_notice;
  hub-core part handed off (message 69fc387c).
- T05: GET /messages/notices, standing_notices in /state/summary,
  dashboard standing-notices panel.
- T06: 17 new tests; full suite green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 63291@bnt-lap001
Assistant-Session: 8bd77868-ca68-4f49-bb1e-d539ecc0d703

migrations/versions/d7e8f9a0b1c2_broadcast_receipts_and_notices.py

@@ -0,0 +1,92 @@
+"""per-recipient broadcast receipts and standing notices (STATE-WP-0093)
+
+Creates ``message_receipts`` (per-reader delivered/read/acknowledged state for
+broadcasts) and adds ``kind``, ``expires_at`` and ``supersedes_id`` to
+``agent_messages``. Existing broadcasts are backfilled to ``kind='news'`` and,
+per the founder decision on D6 (2026-09-22), archived so that none of them
+re-surfaces once broadcast visibility stops depending on the global
+``read_at``. Direct messages are not touched.
+
+Downgrade drops the table, index and columns. Archive stamps stay (they are
+reversible through the API and harmless under the old global read state).
+
+Revision ID: d7e8f9a0b1c2
+Revises: c6f7a8b9d0e1
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = "d7e8f9a0b1c2"
+down_revision = "c6f7a8b9d0e1"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "agent_messages",
+        sa.Column(
+            "kind",
+            sa.String(length=20),
+            nullable=False,
+            server_default="message",
+        ),
+    )
+    op.add_column(
+        "agent_messages",
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.add_column(
+        "agent_messages",
+        sa.Column(
+            "supersedes_id",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey(
+                "agent_messages.id",
+                ondelete="SET NULL",
+                name="fk_agent_messages_supersedes_id",
+            ),
+            nullable=True,
+        ),
+    )
+    op.create_index(
+        "ix_agent_messages_to_kind_archived",
+        "agent_messages",
+        ["to_agent", "kind", "archived_at"],
+    )
+    op.create_table(
+        "message_receipts",
+        sa.Column(
+            "message_id",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("agent_messages.id", ondelete="CASCADE"),
+            primary_key=True,
+        ),
+        sa.Column("agent", sa.String(length=100), primary_key=True),
+        sa.Column("delivered_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("read_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("acknowledged_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index("ix_message_receipts_agent", "message_receipts", ["agent"])
+
+    # D6: existing broadcasts become news and are archived (only broadcasts).
+    op.execute(
+        "UPDATE agent_messages SET kind = 'news' WHERE to_agent = 'broadcast'"
+    )
+    op.execute(
+        "UPDATE agent_messages SET archived_at = now() "
+        "WHERE to_agent = 'broadcast' AND archived_at IS NULL"
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_message_receipts_agent", table_name="message_receipts")
+    op.drop_table("message_receipts")
+    op.drop_index("ix_agent_messages_to_kind_archived", table_name="agent_messages")
+    op.drop_constraint(
+        "fk_agent_messages_supersedes_id", "agent_messages", type_="foreignkey"
+    )
+    op.drop_column("agent_messages", "supersedes_id")
+    op.drop_column("agent_messages", "expires_at")
+    op.drop_column("agent_messages", "kind")