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

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

This branch (sagitta, 1.4.x previous LTS) is migrated to MyST
Markdown except for two pages with known converter issues that
stayed in RST: docs/cli.rst and docs/installation/cloud/aws.rst.
source_suffix in docs/conf.py is `['.rst', '.md']` to cover both.

Changes:

- README.md: new `## Source format` section naming the two stuck
  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 name the two 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.

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

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