docs: add user-facing behavior test check to agents guide

[rlippmann]

what changed

• Added a new Test coverage expectations section to AGENTS.md.
• Added a pre-PR check to confirm whether a change affects user-facing behavior and whether tests cover it.
• Listed user-facing areas to evaluate: engine decisions/prompts, checkpoint continuation behavior, clarify confirmation flows, LiteLLM/OpenWebUI integrations, and integration error-path normalization.
• Added guidance to add or update tests when user-facing behavior changes and to avoid relying only on coverage metrics.

why the change was needed

• This makes test expectations explicit in contributor guidance so user-facing behavior changes are consistently protected by targeted tests before PRs are opened.