[fengnanrui]

Summary

Fixes #1018.

initial_schema builds current ORM metadata, so empty databases already contain
the Feishu delivery_target_id columns. Revision f065 used to add them again
unconditionally, breaking fresh installations and Drone build 588.

Inspect both tables independently and add/drop only absent/present columns.
Existing UUID values are preserved on upgrade, partial schemas are handled,
and inspection failures remain errors. No revision ID, graph edge, business
data or dependency declaration changes. A later revision cannot repair this
failure because bootstrap stops at f065; the guards must live in that revision.
The owning Agent Note documents the rationale and rollback semantics.

Verification

• 13 of the 16 new cases fail before the fix, including real PostgreSQL duplicate-column errors; all 16 pass after it.
• CLAWITH_TEST_POSTGRES_BIN=<PostgreSQL 17 bin> .venv/bin/python -m pytest tests/test_feishu_group_target_migration.py tests/test_agent_model_deleted_at_migration.py -q: 21 passed.
• Independently verified the complete empty-database upgrade, current --check-heads, downgrade one revision and re-upgrade on PostgreSQL 15 as used by CI. Both resulting columns are nullable UUIDs.
• The opt-in PostgreSQL tests create and clean up their own Unix-socket cluster with TCP disabled. No existing database or production server is used.
• Focused Ruff, git diff --check, Gitleaks and the unchanged single Alembic head check pass.

Fork checkout in the upstream Drone pipeline is separately addressed by #1015;
this PR deliberately does not duplicate that change. Once checkout is fixed,
the existing fresh-database CI stage exercises this migration repair.
The documented pre-push skill and docs/testing.md are absent from upstream;
available checks were run directly. No production deployment was performed.

Checklist

• Tested locally
• No unrelated changes included