chore(ai): deprecate @superdoc-dev/ai (#2939)

* chore(ai): mark @superdoc-dev/ai deprecated, remove from docs

Mark packages/ai/package.json as private and strip publish/release
scripts so the package can never be accidentally republished.

Add a deprecation banner to packages/ai/README.md pointing users at
@superdoc-dev/mcp and @superdoc-dev/sdk with the Document API.

Delete the already-hidden apps/docs/ai/ai-actions/ pages and remove
@superdoc-dev/ai entries from apps/docs/scripts/validate-code-imports.ts
and apps/docs/__tests__/lib/extract.ts so the docs stop advertising
(and stop lint-allowing) a deprecated package.

Run npm deprecate '@superdoc-dev/ai@*' '<message>' alongside merging
this PR to emit install-time warnings for existing consumers. Full
source removal (rm -rf packages/ai, pnpm-workspace.yaml entry, eval
references, scripts/type-check-all and scripts/test-cov entries) is
tracked as a separate cleanup PR.

* docs: remove ai-builder placeholder page

ai-builder/overview.mdx only existed to link to the now-deleted
ai-actions docs and was already hidden from navigation. Remove the
page and drop the corresponding skip pattern from the docs test
extract helper so no broken internal links remain.

Author: caio-pizzol

apps/docs/__tests__/lib/extract.ts

@@ -20,8 +20,6 @@ const SKIP_FILE_PATTERNS = [
   /document-api\//,
   /solutions\/esign\//,
   /solutions\/template-builder\//,
-  /ai\/ai-actions\//,
-  /ai\/ai-builder\//,
   /getting-started\/frameworks\//,
   /snippets\//,
 ];
@@ -34,7 +32,6 @@ const SKIP_IMPORTS = [
   'hocuspocus',
   'fastify',
   'express',
-  '@superdoc-dev/ai',
   '@superdoc-dev/esign',
   '@superdoc-dev/template-builder',
   '@superdoc-dev/superdoc-yjs-collaboration',