fix(invariants): align InvariantModel with codebase ULID and timestamp standards

InvariantModel used String(36) UUID-style PK instead of the codebase-
standard ULID (String(26)), and was missing the required updated_at
column present on all peer entity models.  Updated both the ORM class
and the migration DDL to match conventions:

- PK: String(36) → String(26) for ULID compatibility with resource_id,
  decision_id, checkpoint_id, job_id, etc.
- Added updated_at column (String(30)) matching NamespacedProjectModel,
  ResourceLinkModel and other entity tables
- Updated migration `m3_001_invariants_table` DDL to mirror ORM model
- Added index on updated_at for audit-query performance

src/cleveragents/infrastructure/database/migrations/versions/m3_001_invariants_table.py

@@ -22,12 +22,17 @@ depends_on: str | Sequence[str] | None = None
 
 
 def upgrade() -> None:
-    """Create the invariants table with index on is_active."""
+    """Create the invariants table with indexes on is_active and updated_at.
+
+    The ``id`` column uses ULID (26-char string) for consistency with all
+    other models in the codebase (resource_id, decision_id, etc.).
+    """
     op.create_table(
         "invariants",
-        sa.Column("id", sa.String(36), nullable=False),
+        sa.Column("id", sa.String(26), nullable=False),
         sa.Column("description", sa.Text, nullable=False),
         sa.Column("created_at", sa.String(30), nullable=False),
+        sa.Column("updated_at", sa.String(30), nullable=False),
         sa.Column(
             "is_active",
             sa.Boolean,
@@ -38,9 +43,11 @@ def upgrade() -> None:
         sa.CheckConstraint("description != ''"),
     )
     op.create_index("ix_invariants_is_active", "invariants", ["is_active"])
+    op.create_index("ix_invariants_updated_at", "invariants", ["updated_at"])
 
 
 def downgrade() -> None:
     """Drop the invariants table."""
+    op.drop_index("ix_invariants_updated_at", table_name="invariants")
     op.drop_index("ix_invariants_is_active", table_name="invariants")
     op.drop_table("invariants")

src/cleveragents/infrastructure/database/models.py

@@ -1315,8 +1315,9 @@ class InvariantModel(Base):  # type: ignore[misc]
     """SQLAlchemy database model for global invariants.
 
     Stores user-defined constraint rules that must hold true across all
-    planning sessions.  Each invariant carries a UUID identifier, a human-
-    readable description, and an active flag for soft-deletion (is_active).
+    planning sessions.  Each invariant carries a ULID identifier, a human-
+    readable description, an active flag for soft-deletion (is_active), and
+    audit timestamps.
 
     Mapped to table ``invariants`` (migration ``m3_001_invariants_table``).
     """
@@ -1324,9 +1325,12 @@ class InvariantModel(Base):  # type: ignore[misc]
     __allow_unmapped__ = True
     __tablename__ = "invariants"
 
-    id = Column(String(36), primary_key=True)
+    # PK: ULID (26-char string) — consistent with all other models in the
+    # codebase (resource_id, decision_id, checkpoint_id, job_id, etc.).
+    id = Column(String(26), primary_key=True)
     description = Column(Text, nullable=False)
     created_at = Column(String(30), nullable=False)
+    updated_at = Column(String(30), nullable=False)
     is_active = Column(
         Boolean,
         nullable=False,
@@ -1340,6 +1344,8 @@ class InvariantModel(Base):  # type: ignore[misc]
         return (
             f"InvariantModel(id={self.id!r}, "
             f"description={self.description!r}, "
+            f"created_at={self.created_at!r}, "
+            f"updated_at={self.updated_at!r}, "
             f"is_active={self.is_active})"
         )