ci: inline doc-linter on circinus to match rolling

The lint workflow on this branch referenced
`vyos/.github/.github/workflows/lint-doc.yml@current`, which no
longer exists in `vyos/.github` — `lint-doc.yml` was dropped from
the central workflow set during the move toward repo-local linters.
The reusable-workflow `uses:` line has been silently failing
(no `doc-lint` check on recent circinus PRs).

Mirror rolling's approach (inlined 2026-05-10):

- Add `scripts/doc-linter.py` in this repo. Sourced from PR #2014's
  HEAD on rolling (the version that includes the `is_docs_path()`
  docs/-only scope guard so repo-root meta files like AGENTS.md and
  README.md are out of scope, plus the realpath / narrow-exception /
  Ruff fixes from that PR's review pass).
- Replace `.github/workflows/lint-doc.yml` with an in-repo workflow
  that runs `python scripts/doc-linter.py "$FILES_MODIFIED"` against
  the changed-file list from `trilom/file-changes-action`. Same job
  shape as rolling.
- Update the `## Lint` and `## CI` bullets in AGENTS.md to point at
  the in-repo paths.

This restores actual lint coverage on circinus PRs and keeps the
toolchain identical across rolling / circinus / sagitta. Behavior on
the 11 remaining `.rst` pages is unchanged — the linter's
SUPPORTED_EXTS already covers `.md`, `.rst`, and `.txt`.

🤖 Generated by [robots](https://vyos.io)

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