Plan / Interface: session configuration generations

[jhrozek]

Summary

• define explicit replacement intent for model and reasoning-effort switches
• group replacements through one exact predecessor reference without a durable chat root
• activate replacement visibility only after server-side target readiness
• keep generation-aware paging bounded, owner-filtered, fail-visible, and compatible with legacy stores and drivers
• preserve every physical session and all non-main inventory rows

Interface review

This is the Plan / Interface PR. Implementation starts only after this contract is reviewed and merged.

Verification

• task docs
• acceptance-plan checker
• strict documentation and link checks
• git diff --check

Ref: #1490