[Fix] Fast conversations lose context after cold starts

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote or in the web UI.

What changed

• Persist the native OpenCode session ID on each durable Fast conversation.
• Validate and resume persisted sessions after process restarts or local cache eviction, sending only the new turn when the session remains usable.
• Keep visible-text transcript reconstruction as a last-resort fallback when OpenCode no longer has the persisted session.
• Preserve native sessions across provider retries and terminal failures, reconcile stale process-local state against newer durable state, and log the selected session path.

Why this change was made

Fast conversations previously flattened visible history into a synthetic prompt whenever their process-local session cache was cold. That discarded native tool calls, tool results, reasoning blocks, provider signatures, and cache prefix continuity even when OpenCode still had the original session.

Impact

Fast conversations retain their full native transcript across cold process-local starts when the deployment preserves OpenCode storage. Deployments with ephemeral or unavailable OpenCode storage safely validate the persisted ID and fall back to the existing compatibility-history reconstruction instead of failing the turn.

[roomote-community]

No new code issues found. See task

_{Reviewed 27a2eaf}