chore(#597): replace count-based ratchet guards with AST lint + named-set allowlists (#603)

The windows-test-parity ratchet greps test source for fs.rmSync-without-
maxRetries (and six other Windows-portability anti-patterns), failing when an
integer offender COUNT exceeds a frozen baseline (rmSync: 95). A count ratchet
is a Goodhart metric: fixing one offender and adding another keeps the count
constant, so a new defect slips through green. Replace it — and every other
count ratchet in the repo — with a layered, masking-proof design.

Behavioral seam test
- tests/helpers-cleanup.test.cjs proves helpers.cleanup() carries the Windows
  EBUSY retry budget. cleanup() delegates retries to Node's fs.rmSync via
  maxRetries (it owns no loop), so the test asserts the option contract
  (recursive/force/maxRetries>0/retryDelay>0) + real-FS removal + the cwd-guard,
  rather than a loop that does not exist. The EBUSY risk is now tested ONCE at
  the helper, not approximated textually at every call site.

Write-time ESLint rule (AST-accurate, replaces the grep)
- eslint-rules/no-raw-rmsync-in-tests.cjs (error in tests/**/*.test.cjs) bans
  raw fs.rmSync, steering to cleanup(). Catches member, computed (fs['rmSync']),
  destructured and aliased forms; escape hatch is inline
  `// eslint-disable-next-line local/no-raw-rmsync-in-tests -- <reason>` only.
- Migrated 336 raw fs.rmSync teardown calls across ~116 test files to cleanup().
  ~18 genuinely load-bearing sites (mid-test SUT/fault-injection removals,
  error-swallowing or name-colliding local teardown helpers) keep the raw call
  with an inline eslint-disable + reason.

Shared anti-ratchet primitive
- scripts/lib/allowlist-ratchet.cjs:
  - assertWithinAllowlist: fails on NOVEL ids (new offender introduced) AND on
    STALE ids (a known offender was fixed but not pruned) — identity, not count,
    and a ratchet DOWN toward zero.
  - assertTightCeiling: a size/length budget whose ceiling must stay within a
    grace band of the high-water mark, so budgets may only tighten, never creep.

Ratchets converted onto the primitive
- windows-test-parity-guard.test.cjs: rmSync rule deleted (now ESLint-enforced);
  the remaining six patterns moved from integer baselines to named-set
  allowlists with ratchet-down.
- scripts/lint-test-file-count.{cjs,allowlist.json}: per-module integer counts →
  named filename sets (closes the swap-a-file-keep-the-count blind spot); a
  module dropping under cap now FAILS to force pruning its allowlist entry.
- enh-2790 skill-count `<= 63` → named skill allowlist (ratchets toward ~58).

Size budgets hardened (tighten-only)
- agent-size / workflow-size / feat-3039 help-tiered: ceilings lowered to the
  current high-water mark and an assertTightCeiling anti-creep check added per
  tier. Fixed external-contract limits (description ≤100 chars, agent ≤100 KB)
  are intentionally left as-is — they are not grandfathered creeping budgets.

No user-facing behavior change (tests + tooling only); no USER_FACING_PREFIXES
touched, so no changeset fragment is required.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

eslint.config.mjs

@@ -8,16 +8,18 @@ import { fileURLToPath } from 'url';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 
-// Local plugin with three custom AST rules
+// Local plugin with custom AST rules
 import noSourceGrep from './eslint-rules/no-source-grep.cjs';
 import noMagicSleepInTests from './eslint-rules/no-magic-sleep-in-tests.cjs';
 import noElapsedAssertion from './eslint-rules/no-elapsed-assertion.cjs';
+import noRawRmsyncInTests from './eslint-rules/no-raw-rmsync-in-tests.cjs';
 
 const localPlugin = {
   rules: {
     'no-source-grep': noSourceGrep,
     'no-magic-sleep-in-tests': noMagicSleepInTests,
     'no-elapsed-assertion': noElapsedAssertion,
+    'no-raw-rmsync-in-tests': noRawRmsyncInTests,
   },
 };
 
@@ -108,6 +110,8 @@ export default tseslint.config(
       // Timing anti-patterns — warn for now; flip to error after cleanup
       'local/no-magic-sleep-in-tests': 'warn',
       'local/no-elapsed-assertion': 'warn',
+      // Ban raw fs.rmSync in tests — use helpers.cleanup() for Windows-EBUSY retry budget
+      'local/no-raw-rmsync-in-tests': 'error',
       // Ban raw setTimeout sync + elapsed/duration-style assertions via no-restricted-syntax
       'no-restricted-syntax': [
         'warn',