[codex] Add external snippet provenance comments

[danielporterda]

What changed

Adds a searchable source-snippet HTML comment to each generated external snippet MDX file. The comment records the source repo, source path, source commit hash, snippet name, and extraction location.

The generator keeps import-leading MDX valid by placing the provenance comment after the import block when a generated snippet starts with import statements.

Also documents the provenance convention in the external snippet update workflow README.

Why

This creates a stable text-search join between generated snippet files and the source repository state that produced them, which is the first step toward auditing orphaned or stale external snippets.

Validation

• node --check scripts/generateOutputDocs.js
• node --check scripts/helpers/generateOutputDocs.js
• npm run generate:external-snippets -- --list
• git diff --check origin/main...HEAD
• Temporary git-backed extraction smoke test verified the comment includes the source commit and preserves import-first MDX ordering.

Notes

This PR changes the generator and docs only. It does not regenerate the checked-in snippet corpus yet.