agent doc: add a requirement for PR communications (#14709)

* modular guide: helpers take explicit arguments, not `components`

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

* agent guide: human-in-the-loop rules, helpers take explicit arguments

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

* clarify human-in-the-loop scope to communication

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

* contribution guide + agent guide: PR communication must be concise and human-readable

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

* Apply batched suggestions from code review

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Apply suggestion from @yiyixuxu

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>