chore: remove RST swap mechanism, archive rst-*.rst under docs/_rst_legacy/

The swap mechanism (RST-as-fallback for migrated MD pages) is dormant —
docs/_rst_overrides.txt has been empty since the MyST flip trio
landed. The mechanism's surface area is dead weight and the rst-*.rst
shadows scattered across the source tree cause Context7's parser to
misclassify the project as RST.

Sibling PRs:
- yuriy/remove-rst-swap-mechanism (rolling)
- yuriy/remove-rst-swap-mechanism-circinus

Changes:
- Move 210 rst-*.rst shadow files into docs/_rst_legacy/ preserving
  subdirectory structure. They remain in the repo for reference; Sphinx
  excludes the folder via exclude_patterns.
- Strip swap_sources.py invocation from docs/Makefile.
- Strip rst-*.rst exclude entry and the _md_exclude.txt loader from
  docs/conf.py; replace with a single _rst_legacy exclude.
- Delete scripts/swap_sources.py, tests/test_swap_sources.py,
  docs/_rst_overrides.txt.
- Update AGENTS.md: drop the "RST override mechanism" section and the
  test-runner snippet for the deleted test.

Note: .readthedocs.yml on sagitta has no jobs: block to remove (the
swap was wired only at build-time via the Makefile chain on this branch).

Verified: sphinx-build -b html with --keep-going produces identical
warning set (409 unique — pre-existing cli.rst/aws.rst title-level
warnings on this branch), identical sitemap entry count (215),
identical llms.txt entry count (23), zero rst-* URLs in any artifact.

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

0 files changed, 0 insertions, 0 deletions