docs: keep notices fresh at commit time instead of a new CI gate

Regenerate THIRD_PARTY_NOTICES.md from a pre-commit job whenever a
manifest, lock file, vendor manifest, or pyproject is staged, and assert
the committed bytes inside the generator spec the test lane already runs.
Drops the separate doc-sync gate: no extra CI process, and a dependency
edit no longer bounces back from CI to rerun a generator.

scripts/gen-third-party-notices.spec.ts

@@ -1,10 +1,20 @@
 import { readFileSync } from 'node:fs'
 import { resolve } from 'node:path'
 import { describe, expect, it } from 'vitest'
-import { type Manifest, parseVendoredRows, tierExternalDeps } from './gen-third-party-notices.ts'
+import { type Manifest, parseVendoredRows, render, tierExternalDeps } from './gen-third-party-notices.ts'
 
 const root = resolve(import.meta.dirname, '..')
 
+describe('THIRD_PARTY_NOTICES.md', () => {
+  // Freshness lives here rather than in its own doc-sync gate: this spec file
+  // already runs in the test lane, so the check costs no extra CI process.
+  // Pre-commit regenerates the file whenever a manifest is staged, so reaching
+  // this assertion means the notices were committed without that hook.
+  it('matches what the generator produces from the current manifests', () => {
+    expect(readFileSync(resolve(root, 'THIRD_PARTY_NOTICES.md'), 'utf8'), 'stale notices — run `pnpm run gen-third-party-notices`').toBe(render())
+  })
+})
+
 /** Build the (manifests, names) pair `tierExternalDeps` consumes. */
 function workspace(entries: Record<string, Manifest>): { manifests: Map<string, Manifest>; names: Set<string> } {
   const manifests = new Map(Object.entries(entries))