[chore] logical-gaps: a2a service hierarchy gate (typed, unconditional) + persist skill on message row (3 gaps)

create_a2a_notification gated A2A hierarchy only when both ends resolved
(`if from_agent and target_agent:`), so an unattributed (from_agent falsy)
or unresolvable-target request slipped past the hierarchy matrix and
dispatched with from_agent='unknown' / to_agent='' — and a denial came back
as a bare ValueError indistinguishable from the missing-task_id ValueError.
Require both ends present, then validate via the shared typed
validate_a2a_access path (A2AAccessDeniedError + route_hint) so the legacy
notification surface enforces the same who-may-talk-to-whom invariant as the
conversation path.

send() accepts skill= and the gateway callers (qa/doc/pr_gate) pass it
expecting the receiver to learn which capability the message is about, but
send_chat_message never read it from options — silently dropped. Persist a
nullable skill column (migration 054) on a2a_messages, wire it through
send_chat_message + _msg_to_model + the A2AChatMessage model, and fix the
send() docstring (it claimed 'recorded in message metadata').

TDD: 4 red→green (skill recorded on message + surfaces in inbox; permission
denied raises typed A2AAccessDeniedError with route_hint; self-A2A raises
typed; missing from_agent raises instead of silent dispatch). 103 a2a
integration tests green; ruff/mypy clean; migration 054 verified
upgrade/downgrade on throwaway PG.

alembic/versions/054_a2a_message_skill.py

@@ -0,0 +1,32 @@
+"""Persist ``skill`` on a2a_messages.
+
+Directed A2A ``send()`` accepts a ``skill=`` (the capability the sender is
+exercising/requests of the receiver, e.g. ``code_review``) and the gateway
+callers (qa / doc / pr_gate) pass it expecting the receiver to learn which
+capability the message is about. Until now ``send_chat_message`` never read it
+from options, so it was silently dropped — the receiver's inbox showed a bare
+message with no skill context. This adds a nullable ``skill`` column so the
+capability rides on the row and surfaces in the inbox model.
+
+Revision ID: 054_a2a_message_skill
+Revises: 053_playbook_archived_attr
+Create Date: 2026-06-30
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "054_a2a_message_skill"
+down_revision = "053_playbook_archived_attr"
+branch_labels: dict[str, str] | None = None
+depends_on: dict[str, str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column("a2a_messages", sa.Column("skill", sa.String(100), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("a2a_messages", "skill")