Write the versioning policy into AGENTS.md #323

Two minors shipped breaking changes: 1.11.0 dropped Python 3.10 (chore!, #270), and 1.12.0 moved Litestar tracing to Litestar's OpenTelemetryPlugin (feat!, #279), with a "Breaking changes" section in its release notes. Nothing in the repo says when a change needs a major. AGENTS.md's alias rule (AGENTS.md:51-53) makes every rename a silent alias and gives no path for removing one.

Work

• Add a short versioning section to AGENTS.md:
  • what counts as breaking, for example: a user's working code or deployed behaviour changes without them editing anything. That covers new defaults, removed names, raised floors and a changed HTTP surface;
  • breaking means a major release;
  • whether dropping an end-of-life interpreter is an exception. 1.11.0 treated it as one, and modern-python/lite-bootstrap#272 plans the same for 3.11. Write down whichever is decided;
  • how a rename retires: the alias warns with DeprecationWarning for at least one minor and is deleted in the next major (this replaces the "silent alias" wording; see Remove deprecated aliases, after a 1.x release that warns about them #314);
  • breaking changes get a "Breaking changes" section in the release notes, as 1.12.0 has.
• Keep it to rules an agent cannot infer from the code, per the existing "Code style" framing.

Verify: just adr-check and the links job pass, and every line of the new section states a rule.