docs: refresh README + AGENTS source-format claim for circinus

Adapted from PR #2014 on rolling, tailored to this branch's actual
state.

This branch (circinus, 1.5.x LTS) is mostly migrated to MyST
Markdown. 11 RST pages remain in the build alongside the .md set:

- docs/changelog/index.rst + docs/changelog/{1.2.1..1.2.6, 1.3,
  1.4, 1.5}.rst (auto-generated; stayed RST)
- docs/automation/terraform/terraformvyos.rst

source_suffix in docs/conf.py is `['.rst', '.md']` to cover both.

Changes:

- README.md: new `## Source format` section naming the legacy RST
  pages, pointing at the `_rst_legacy/` archive, and noting the
  MyST-fence form for VyOS command directives in .md pages. The
  `## Contributing` blurb drops the now-imprecise "MyST for migrated
  pages, RST for the rest" phrasing.
- AGENTS.md: `## Project` and `### Source files` sections updated
  to enumerate the 11 remaining RST pages explicitly so contributors
  know which files are still RST and which to write as MyST.

Linter origin claim ("doc-linter.py from the vyos/.github repo")
left intact — that script is NOT inlined on this branch (the inline
move on rolling at 2026-05-10 has not been backported to circinus).

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

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