S2: harvest — post-merge librarian distills a provenance-stamped skill card (skill-tree #458)

[hadamrd]

Part of #458, depends on S1. The leaf-adding mechanism.

Acceptance

• After a PR merges critic-clean, a librarian sub-agent diffs what the worker did and writes/updates a skill card for the touched area, stamped with the merged SHA + verified date.
• Updating an existing card for the same area+trigger does NOT duplicate — it revises (check-before-write).
• Adversarial: a critic-BLOCKED or reverted PR produces NO card (only proven procedure is harvested); removing that gate turns a test red. A second merge in the same area revises the one card rather than creating a duplicate.