fix(alembic): bridge ghost migration 2026_05_28_add_tool_definition_manifests

The production database was stamped with a migration that no longer exists
in the codebase (created on another branch, applied, then removed). This
adds a no-op bridge migration so Alembic can reconcile the DB state.

- Create bridge migration 2026_05_28_add_tool_definition_manifests (no-op)
- Re-chain terminal_sessions migration to depend on the bridge
- Fixes startup failure: Can't locate revision identified by ...

apps/api/alembic/versions/2026_05_28_add_terminal_sessions_table.py

@@ -6,16 +6,16 @@ Create Date: 2026-05-28
 
 """
 
-from typing import Sequence, Union
+from collections.abc import Sequence
 
 from alembic import op
 import sqlalchemy as sa
 
 # revision identifiers, used by Alembic.
 revision: str = "2026_05_28_add_terminal_sessions"
-down_revision: Union[str, None] = "20260527_160017_add_pi_agent"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
+down_revision: str | None = "2026_05_28_add_tool_definition_manifests"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
 
 
 def upgrade() -> None:

apps/api/alembic/versions/2026_05_28_add_tool_definition_manifests.py

@@ -0,0 +1,33 @@
+"""bridge: add_tool_definition_manifests (ghost migration)
+
+Revision ID: 2026_05_28_add_tool_definition_manifests
+Revises: 20260527_160017_add_pi_agent
+Create Date: 2026-05-28
+
+This is a no-op bridge migration. The original migration was created on
+another branch and applied to the production database, but the file was
+subsequently removed. This bridge allows Alembic to reconcile the DB
+state with the local revision history.
+
+"""
+
+from collections.abc import Sequence
+
+# revision identifiers, used by Alembic.
+revision: str = "2026_05_28_add_tool_definition_manifests"
+down_revision: str | None = "20260527_160017_add_pi_agent"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    # No-op: this migration was already applied on the target database.
+    # If the schema was actually changed by the original migration,
+    # those changes must be preserved manually or via a separate migration.
+    pass
+
+
+def downgrade() -> None:
+    # No-op: cannot safely reverse a migration whose original operations
+    # are unknown.
+    pass