test(build): enforce @ai specificity (denylist + duplication cap)

[netravnen]

Make the ratified @ai anti-boilerplate rules (wayfinder ticket
ai-enforcement-parameters, 2026-08-20) machine-checked so stock tags
cannot be reintroduced. The insight that makes this enforceable without
defining "boilerplate": boilerplate is, by definition, repeated -- so
an exact-duplication cap plus a denylist of the historical stock
phrases covers the space with no similarity scoring.

Changes:

• tests/conventions-ai-tags.test.js: pure node:test + fs conformance
  test over all six source fragments (three .src.js plus
  lib/admincom-common.js, lib/cp-name-normalization.js,
  lib/admincom-entity-exclusions.js). Three assertions: the seven
  historical stock phrases are denylisted exactly; identical
  normalized @ai text may appear at most 2 times repo-wide (permits
  one deliberate twin pair, blocks templates); tag prose must carry
  at least 30 characters of substance.
• docs/CONVENTIONS.md "Documentation convention": rewritten to the
  ratified rule -- an @ai tag states one invariant specific to that
  function that a plausible diff could silently violate, with the
  litmus sentence ("If the sentence could be pasted onto a different
  function unchanged and still read true, it is not an @ai tag");
  drops the stock-phrase examples and the "900+ occurrences" text.

Security:

• N/A

Testing:

• Non-vacuity proven: temporarily re-adding the stock tag "Keep
  behavior stable and prefer minimal, localized edits." to
  lib/admincom-common.js turned the denylist assertion red with the
  exact file:line, then the file was restored and the suite went
  green again.
• The one existing exact-duplicate pair (DP's two "Preserve menu
  command registration behavior." tags) sits at the N=2 cap and
  passes by design.
• python user.js/scripts/build_userscripts.py && --check: green.
• node --check on all three generated .user.js: green.
• cd user.js && node --test: 589 pass, 1 skipped, 0 fail
  (585 baseline + 4 new).

Backwards Compatibility:

• Test and docs only; no script behavior change.

Assisted-by: Claude:claude-opus-5

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