fix(ci): stop validate failing on stale generated_at date

[castrojo]

Problem

The validate check has been producing false-negative failures across the open PR backlog. At time of writing, 9 of 19 open PRs (#893, #917, #924, #926, #929, #931, #932, #933, #934) showed a red validate check unrelated to their own content:

error: docs/skills/index.json is stale. Run scripts/generate_skill_index.py --write

Root cause

scripts/generate_skill_index.py stamped generated_at with date.today() at script-execution time, then byte-compared the result against the committed docs/skills/index.json.

That means the check only passed if CI happened to run on the same calendar day the index was last regenerated on main — entirely independent of whether any skill doc actually changed. Any PR whose CI ran a day or more after the last regeneration failed.

Reproduced locally with zero content edits: --check failed purely because days had passed since the last commit to index.json.

This matters because validate is a required status check in the merge queue ruleset, so the noise trains reviewers to ignore a gate that is supposed to block merges.

Fix

Only advance generated_at when the catalog's actual content (schema_version + skills) differs from what is committed. --write becomes idempotent when nothing changed; --check tolerates a stale date as long as the skill data matches.

Verification

Proved in both directions, with a regression test for each:

• No false negative — backdated the committed index to 2020-01-01 with zero skill content changes; --check now passes (previously failed). Covered by test_generate_skill_index_check_tolerates_stale_generated_at.
• Real drift still caught — edited a skill's front matter without regenerating; --check still fails with index.json is stale. Covered by test_generate_skill_index_check_still_fails_on_real_drift.

just check passes. Skill doc test suite: 8 passed.

Alternatives rejected

• Compare against the merge result instead of PR head — doesn't address the cause; the merge tree still gets stamped "today".
• Scope the check to changed files — would blind it to a PR that edits a skill doc without regenerating the index.
• Stop committing generated files — far larger blast radius for a date-comparison bug.
• Auto-commit bot step in CI — added complexity over a small fix.

Assisted-by: Claude Sonnet 5 via GitHub Copilot