docs(rfd): propose session-scoped instructions

[CrysisDeu]

Summary

• propose a first-class sessionInstructions field on NewSessionRequest
• let agents advertise append and replace support independently
• define instructions as session state restored by load/resume and inherited by fork
• keep agent policy and enforced controls outside replaceable instructions

Scope

This pull request adds the RFD and its Draft navigation entry. It does not change the protocol schema or SDKs.

Questions for discussion

• Is sessionInstructions the right agent-level name?
• Should instructions remain creation-only session state?
• Should the first version include both append and replace?