[ACP on Cloud] Backend: native session/load resume integration (id mirror + snapshot + precedence)

[simonrosenberg]

Priority: P1. Part of #988. The OpenHands-side of Tier-B native session/load resume — companion to the SDK files-half (#1126 = software-agent-sdk#3562) and the persistence substrate (#1018). Gives a first-class home to the work that replaced the closed #14506 (id-half) and #14311.

Implemented by OpenHands#14709

• Mirror acp_session_id / acp_session_cwd / acp_agent_version from the agent_state webhook stream onto conversation_metadata (app_server migration 012 / enterprise 119).
• AcpSessionSnapshotService: pull the allowlisted CLI session blob into config.file_store at turn boundaries; reseed it into a fresh sandbox on rebuild.
• _build_acp_start_conversation_request: attempt native resume first (mirrored id + session/load-capable provider + matching cwd + restorable blob → set acp_resume_session_id + acp_isolate_data_dir), degrading to #14640 bootstrap otherwise.

Tasks

• Review + merge OpenHands#14709 (CI-green, e2e-validated; blocked only on human review).
• Sync alembic migrations before merge.
• Land the secret-free spec snapshot ([ACP on Cloud] Cloud: carry the ACP agent spec (snapshot agent_settings at creation) #1015 / OpenHands#14671) alongside, so resume rebuilds the right provider.

Done when

• A recycled cloud ACP conversation resumes via native session/load (same acp_session_id, no <<RESUMED CONVERSATION>> marker) for Claude + Codex. (Gemini stays on bootstrap — no reliable session/load.)

Depends on software-agent-sdk#3562 (blob routes) + the runtime-image pin (tracked separately).