Add Alembic migration for work_claims.task_id column

Single nullable VARCHAR(100) column addition. Applied and verified
against the real local Postgres: claims with task_id round-trip
correctly, claims without task_id work unchanged.

backend/alembic/versions/2026_04_14_2000_a8b9cadbef01_add_task_id_to_work_claims.py

@@ -0,0 +1,30 @@
+"""Add task_id column to work_claims.
+
+Allows claims to reference a specific structured task by its id field,
+enabling precise collision detection when multiple agents start
+near-simultaneously.
+
+Revision ID: a8b9cadbef01
+Revises: f7a8b9cadbef
+Create Date: 2026-04-14 20:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = "a8b9cadbef01"
+down_revision = "f7a8b9cadbef"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "work_claims",
+        sa.Column("task_id", sa.String(100), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("work_claims", "task_id")