test(tests): antagonistic tier — fast-check property tests + Stryker mutation testing (PR-gated) (#461)

* feat(#454): add antagonistic tier — fast-check property tests + Stryker mutation config

Adds property-based testing (fast-check v4) and mutation testing scaffolding
(Stryker v8) as the antagonistic validation tier for lib/*.cjs pure logic.

## Files added

### Shared setup
- tests/helpers/fast-check-setup.cjs — configureGlobal({ numRuns:200, seed:42 })
  for deterministic CI; override locally with GSD_FC_SEED

### Property test suites (node:test + fast-check)
- tests/context-utilization.property.test.cjs — boundary at 60%/70% thresholds
  (exact Math.ceil boundary, not Math.floor), TypeError on all invalid inputs,
  overflow clamping to 100%/critical, shape invariants (7 tests, all pass)
- tests/prompt-budget.property.test.cjs — estimateTokens monotonicity + ceil(len/4)
  exactness; applyBudget shape invariant, instructions/roadmap verbatim, budget
  envelope, omit tracking (11 tests, all pass)
- tests/frontmatter.property.test.cjs — extractFrontmatter/reconstructFrontmatter/
  spliceFrontmatter never-throw + type shape + splice→extract round-trip (9 tests)
- tests/adr-parser.property.test.cjs — shouldRejectAdrStatus boundary (3 statuses
  only), parseAdrMarkdown shape + title trim invariant (discovered: parser trims
  trailing whitespace) (9 tests, all pass)
- tests/config-schema.property.test.cjs — isValidConfigKey never throws, returns
  boolean, accepts all VALID/RUNTIME_STATE_KEYS, rejects empty/null/unknown (8 tests)

### Stryker mutation config
- stryker.config.mjs — testRunner:'command', mutate bin/lib/**/*.cjs minus 13
  generated files, coverageAnalysis:'off', thresholds {high:80,low:60,break:50},
  incremental:true, reporters html+clear-text+progress

### CI workflow
- .github/workflows/mutation.yml — PR-gating job (pull_request + workflow_dispatch),
  runs stryker --incremental --since origin/next (changed files only), uploads
  HTML artifact; SINCE_REF via env not interpolation (injection-safe)

### Package config
- package.json: +test:mutation, +test:mutation:since scripts
- .gitignore: +.stryker-tmp/, +.stryker-incremental.json, +reports/mutation/
- package-lock.json: fast-check@4.8.0, @stryker-mutator/core@9.6.1

## Verified
node --test on all 5 property test files: 44 tests, 0 failures.
Stryker NOT run (slow; reserved for CI).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(#454): pin upload-artifact to v7.0.1 SHA used across repo (bad SHA ea165f8d)

The SHA ea165f8d65b6e75b540449d3ec4f5dde0c5a4e1 (labeled v4.6.2) does not
resolve on GitHub Actions. All other workflows in this repo pin
043fb46d1a93c77aae656e7c1c64a875d1fc6a0a (v7.0.1) — align mutation.yml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(#454): mutation workflow — replace invalid --since flag with changed-core-files --mutate scoping

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: CI Rebase Check <ci@gsd-redux>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -49,10 +49,12 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",
+    "@stryker-mutator/core": "^8.7.1",
     "c8": "^11.0.0",
     "eslint": "^9.39.4",
     "eslint-plugin-n": "^17.24.0",
     "eslint-plugin-no-only-tests": "^3.4.0",
+    "fast-check": "^4.8.0",
     "globals": "^16.5.0",
     "js-yaml": "^4.1.1",
     "typescript": "^6.0.3",
@@ -91,6 +93,8 @@
     "test:affected": "node scripts/run-affected-tests.cjs",
     "test:coverage": "c8 --check-coverage --lines 70 --reporter text --include 'get-shit-done/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs",
     "test:coverage:unit": "c8 --check-coverage --lines 70 --reporter text --include 'get-shit-done/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit",
-    "test:coverage:all": "npm run test:coverage"
+    "test:coverage:all": "npm run test:coverage",
+    "test:mutation": "stryker run",
+    "test:mutation:since": "stryker run --incremental --since origin/next"
   }
 }