docs(agents): reflect completed MyST migration + bulk hard-wrap to 80 chars

The previous README refresh sweep landed before the doc-linter was
extended to MyST in 7a3aa29, so AGENTS.md was never wrapped to the
80-char rule its own 'Source conventions' section documents, and the
source-format claims drifted out of sync once the RST->MyST migration
finished.

Content fixes (the inconsistency Copilot flagged on PR #2014):

- '## Project': drop the 'MyST Markdown for migrated pages and RST
  for pages that haven't been migrated yet' framing. State plainly
  that sources are MyST Markdown only, source_suffix is ['.md'],
  and the RST archive under docs/_rst_legacy/ is not editable.
- '### Sphinx config': source_suffix entry updated from
  ['.rst', '.md'] to ['.md'].
- '### Source files': drop the 'docs/<page>.rst' non-migrated bullet
  (no such pages exist). Add a 'docs/_include/<name>.txt' bullet
  noting that the shared snippets stay RST because cmdincludemd
  parses their content as RST.
- '**Editing rules:**': collapse 'Migrated page' / 'Non-migrated
  page' into one 'Existing page' rule. Add a bullet pointing at the
  _include/*.txt -> RST exception.
- '### Command directives': rewrite to lead with MyST fenced blocks
  (the canonical form) and only mention RST forms for {eval-rst}
  blocks and _include/*.txt snippets. Avoid using literal
  quad-backtick code-in-code samples that would otherwise open
  phantom fences in the source.

Linter compliance:

- Hard-wrap every paragraph and bullet outside fenced code blocks
  to <=80 chars (~30 pre-existing violations).
- Wrap the long line about the docutils inline-markup link by
  dropping the link entirely; the rule 'use double backticks' is
  self-contained and AGENTS.md doesn't need the reference.
- Restructure '### Bot review workflow' table: shorten the
  CodeRabbit row to 79 chars and move the 'auto-reviews are
  disabled' note to a paragraph below the table.
- Wrap the '8.8.8.8' real-public-IP bullet under '### IP addresses'
  with '% stop_vyoslinter' / '% start_vyoslinter' markers. The
  surrounding section is literally documenting suppression markers,
  so the meta-example reads naturally.
- The '### Configuration page structure' bullet referring to MD
  fence syntax was using quad-backtick code-in-code; rewritten to
  '`{cfgcmd}` fenced code blocks (in MD)' to avoid opening a
  phantom fence in the linter's view of the source.

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