fix(agent): persist recovered session usage

[devRickyyy]

Summary

• merge provider runtime context recovered during session resume back into the controller session
• emit an explicit session snapshot only when resume recovers new runtime context
• add regression coverage and document the resume data flow and troubleshooting path

Root cause

Provider adapters can replay context-window usage while reattaching to an existing provider session. Controller.Resume previously stored the pre-resume session without reading StateAdapter.SessionState, so the recovered usage remained a live adapter observation.

An event report with an empty event list does not create a state patch, so metadata enrichment cannot make that observation durable. AgentGUI hydration could therefore lose usage after reopening the session.

Impact

Resumed sessions now persist provider-recovered usage and other newly recovered runtime context. Resumes with no context change do not emit an extra report.

Validation

• go test ./runtime -shuffle=on -count=3
• pnpm check:changed (7/7 lanes)
• pnpm check:changed -- --push-ready (8/8 lanes)