* docs: add AGENTS.md and CONTRIBUTING.md
Codifies scenario design guidelines from past PR reviews:
- fewer scenarios with more checks (CI cost multiplies per-SDK)
- same check id for SUCCESS and FAIL, optimize for Ctrl+F
- extend the everything-client/server rather than adding new example files
- prove it passes and fails
- reuse the CLI runner, don't add parallel entry points
Also asks contributors to open an issue for discussion before a PR, and
to target specific spec MUSTs or open issues rather than running generic
AI bug-hunts against the harness.
* docs: broaden issue-first guidance to cover bugs, not just scenarios
