chore(#452): add ESLint 9 flat-config harness with three custom AST rules (#460)

Install eslint@9 + typescript-eslint@8 + globals@16 + eslint-plugin-n@17 +
eslint-plugin-no-only-tests@3 + typescript as devDependencies.

eslint.config.mjs (flat config):
- Global ignores: node_modules, dist, .worktrees, .claude, coverage, the
  12 generated get-shit-done/bin/lib/*.cjs files
- Block for get-shit-done/bin/**/*.cjs + scripts/**/*.cjs: js.recommended +
  eslint-plugin-n + local plugin; generic quality rules (no-var, prefer-const,
  no-unused-vars, no-empty, n/no-process-exit)
- Block for tests/**/*.test.cjs: no-only-tests (error), local timing rules,
  no-restricted-syntax timing bans

eslint-rules/ local plugin (three AST rules, all at warn pending cleanup):
- no-source-grep: flag readFileSync on source .cjs/.js/.ts + text methods
- no-magic-sleep-in-tests: flag Atomics.wait and await-new-Promise(setTimeout)
- no-elapsed-assertion: flag assert*() on timing props (elapsed/duration/took/ms)

tsconfig.lint.json: allowJs + checkJs + noEmit for future type-aware passes.

tests/eslint-rules.test.cjs: 15 RuleTester unit tests (all pass, 0 fail).

package.json: add lint/lint:fix scripts; remove lint:tests (subsumed by ESLint
local/no-source-grep). Rules that produced pre-existing errors downgraded to
warn: no-useless-escape, no-unsafe-finally, no-regex-spaces, no-control-regex,
no-irregular-whitespace. ESLint exits 0 (warnings ok).

.github/workflows/test.yml lint-tests job: add npm ci + ESLint step; remove
"Lint — no source-grep tests" step (now covered by ESLint); bump timeout 3→5
min. .gitignore: add node_modules/.cache/eslint/ entry.

eslint --fix auto-cleaned: no-regex-spaces in tests, prefer-const in state.cjs,
redundant eslint-disable-next-line comments.

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

package.json

@@ -48,8 +48,15 @@
     "ws": "8.20.1"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.4",
     "c8": "^11.0.0",
-    "js-yaml": "^4.1.1"
+    "eslint": "^9.39.4",
+    "eslint-plugin-n": "^17.24.0",
+    "eslint-plugin-no-only-tests": "^3.4.0",
+    "globals": "^16.5.0",
+    "js-yaml": "^4.1.1",
+    "typescript": "^6.0.3",
+    "typescript-eslint": "^8.60.0"
   },
   "optionalDependencies": {
     "fallow": "^2.70.0"
@@ -64,9 +71,10 @@
     "prepublishOnly": "npm run build:hooks",
     "pretest": "npm run lint:skill-deps",
     "pretest:coverage": "npm run lint:skill-deps",
+    "lint": "eslint . --cache --cache-location node_modules/.cache/eslint/",
+    "lint:fix": "eslint . --fix",
     "lint:descriptions": "node scripts/lint-descriptions.cjs",
     "lint:skill-deps": "node scripts/lint-skill-deps.cjs",
-    "lint:tests": "node scripts/lint-no-source-grep.cjs",
     "lint:test-file-count": "node scripts/lint-test-file-count.cjs",
     "lint:pr-checks": "node scripts/lint-pr-check-project-dir.cjs",
     "lint:changeset": "node scripts/changeset/lint.cjs",