docs: add agent PR guidance

[tode-rl]

Summary

• Add root AGENTS.md with repository PR hygiene guidance.
• Add CLAUDE.md as a symlink to AGENTS.md so Claude and Codex-style agents share the same instructions.

Guidance Added

• PR titles and commit subjects must use Conventional Commits format.
• Agents asked to create or update PRs should wait for relevant local validation and passing GitHub CI before handing the turn back, unless blocked by an external issue.
• Generated SDK code should be changed at the OpenAPI/Stainless source when possible, with handwritten changes kept small and separate.

Validation

• git diff --cached --check before commit