Background sessions persist unsent prompt drafts to .prompt-stash

Background sessions now write the in-progress, unsent prompt text (composer text, cursor offset, pasted-content ids, launch-warning state) to a .prompt-stash file under the job directory, validated against a schema and written atomically. A new class subscribes to the prompt-input store's stash field and persists it, and on connection the reverse-proxy server's onConnection now calls promptStash.restore() unconditionally, in addition to the existing restorePromptDraft, so a background session recovers an in-progress but unsent prompt after a restart or reattach. Stash size is capped, falling back to memory-only with a warning if the draft is too large to persist.