feat: remove built-in tool types distinction

- Drop is_builtin column from tool_types table
- Remove built-in tool seeding from startup
- Remove is_builtin from API schemas and frontend types
- Update tool-types spec to reflect removal of built-in concept
- Add Alembic migration for column removal
- Update tests to work without built-in distinction

apps/api/alembic/versions/2026_05_23_remove_is_builtin.py

@@ -0,0 +1,25 @@
+"""remove_is_builtin_from_tool_types
+
+Revision ID: 2026_05_23_remove_is_builtin
+Revises: 2026_05_22_add_clone_mode
+Create Date: 2026-05-23 14:30:00.000000
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = '2026_05_23_remove_is_builtin'
+down_revision = '2026_05_22_add_clone_mode'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Drop the is_builtin column from tool_types
+    op.drop_column('tool_types', 'is_builtin')
+
+
+def downgrade() -> None:
+    # Add the is_builtin column back to tool_types
+    op.add_column('tool_types', sa.Column('is_builtin', sa.Boolean(), nullable=False, server_default='false'))