fix(docs): OKF conformance of developer-docs + CI validation

[galkleinman]

developer-docs/ does not currently pass OKF v0.1 conformance, so tooling that reads the bundle rejects it before reading any of the content.

What fails

Every key on the bundle-root index.md except okf_version. §8 is explicit that index files carry no frontmatter, with exactly one exception — a bundle-root index.md may declare okf_version (§12) — and §11.3 makes the reserved-filename structure a conformance requirement:

developer-docs/index.md
  2  error  `index.md` must not carry frontmatter key `type`
  3  error  `index.md` must not carry frontmatter key `title`
  4  error  `index.md` must not carry frontmatter key `description`
  5  error  `index.md` must not carry frontmatter key `tags`
  7  error  `index.md` must not carry frontmatter key `timestamp`

Nothing is lost by removing them. The body's # developer-docs heading and opening line already carry the title and description, and an index file is not a concept document, so type, tags, and timestamp have no defined meaning on one. The rest of the bundle is already conformant — this is the only file that fails.

This matches developer-docs/documentation-policy.md, which requires frontmatter with a type on concept files and says nothing about index files.

CI

Adds .github/workflows/developer-docs.yaml, running conformance only (§11: parseable frontmatter, non-empty type, reserved-file structure). §11 forbids consumers from rejecting a bundle for broken cross-links, unknown type values, unknown keys, or missing optional fields, so this gate cannot fail on any of those — it catches the structural breakage above and nothing else. The bundle is also clean of advisory warnings today, if you would rather run command: lint instead.

Path-scoped so it runs only when developer-docs/ changes, action pinned by SHA and permissions: contents: read, matching lint.yaml.

The action is okf-toolkit, Apache-2.0. Happy to swap it for a plain curl-and-run step, or to drop the CI commit entirely and keep just the fix, whichever you prefer.

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[gemini-code-assist]

Code Review

This pull request simplifies the frontmatter of the developer-docs/index.md file by removing several metadata fields, including type, title, description, tags, and timestamp, while retaining the okf_version field. There are no review comments, and I have no feedback to provide.

[russellwheatley]

The developer-docs/index.md frontmatter change is correct. OKF §8 only allows okf_version on the bundle-root index, and that matches how the other FirebaseUI bundles are written.

Please drop the workflow commit. We are not going to take galkleinman/okf-toolkit (or a curl of the same binary) as a CI dependency. The frontmatter fix is enough on its own.