Persist /v1/agents/run transcripts (currently response-only)

[jameswnl]

Problem

POST /v1/agents/run returns the execution transcript in the response body but never persists it. Once the response is gone, the transcript is unrecoverable:

• run_agent_handler (src/app/endpoints/agents.py) builds the result dict and returns it — no TranscriptStore save, no filesystem write, no DB row.
• The workflow path (step_runner) saves every step transcript to PostgreSQL (step_transcripts, keyed by (workflow_id, step_name)), readable later via GET /v1/workflows/{workflow_id}/transcripts.
• The query path persists transcripts as filesystem JSON per conversation (store_transcript in src/utils/transcripts.py).

So transcripts have three different fates depending on entrypoint: PG (workflows), files (query), nowhere (agents/run).

Why it matters

• Demos and audits of single-shot agent runs (e.g. the agent-ephemeral Landlock demo) have no server-side record to revisit.
• Consumers must implement their own capture (tee the response) — easy to forget, and inconsistent with the workflow experience.

Design question

There is no natural storage key: step_transcripts is keyed by (workflow_id, step_name) and an ad-hoc agent run has neither. Options:

1. PostgreSQL, new table (e.g. agent_run_transcripts keyed by a generated run id returned in the response) — queryable, consistent with workflows; needs schema + fetch API.
2. Filesystem, mirroring the query-transcript store — cheaper, but a third transcript location with no read API.
3. By design, won't fix — single-shot runs are the caller's to keep; document that the transcript is response-only.

Acceptance

• Decision recorded on options 1–3 above.
• If persisted: transcript retrievable after the fact (API or documented SQL/filesystem path) with a documented retention policy.
• If won't-fix: documented in the endpoint description and the demo script (docs/cloud-agents-demo-curl.sh — e.g. tee the response to a file).
• Related: Transcript detail parity across spawn modes (none/local/ephemeral) in /v1/agents/run #2639 (transcript detail parity across spawn modes).