[Bug] Build agent reads designs from flat files, so orchestrator-run sections can't be built

[sjohnston1972]

Part of the "SQLite migration is half-done" parent issue.

Background

The orchestrator runs a designer/critic loop per section and saves each result to the SQLite section_designs table (backend/app/agents/orchestrator.py, step 3 in _drive_run_inner, writing SectionDesign.record). The build agent turns an approved design into a sandbox topology.

Problem / Goal

backend/app/agents/build.py:210 (converse_build) resolves the design it is about to build by calling get_design(design_id), imported from app.agents.loop. That function (backend/app/agents/loop.py:92) only reads data/designs/<id>.json from disk. Orchestrator designs live in SQLite, not on disk, so there is no way to build a sandbox from an orchestrator run — the core v2 workflow dead-ends.

Goal: let the build flow accept a SectionDesign (SQLite integer id) and read its record from the DB.

Where to look

• backend/app/agents/build.py:190-306 — converse_build, where get_design is called and final_design = iterations[-1]["design"] is extracted.
• backend/app/agents/loop.py:92 — the current flat-file get_design.
• backend/app/db/models.py:82 — SectionDesign; its .record column already holds the exact dict shape the build agent expects ({"iterations": [...], ...}).
• backend/app/api/build.py:37-67 — BuildConverseRequest.design_id and the converse endpoint.
• backend/app/api/runs.py:112 — GET /api/runs/{run_id}/sections/{section_id} shows how a section record is fetched from SQLite.

Suggested approach

1. Add a DB-backed lookup that, given a SectionDesign.id, opens a session (from app.db.session import engine), fetches the row, and returns sd.record (or None).
2. Change converse_build to use that lookup instead of loop.get_design.
3. Decide how design_id is typed on the request: accept the integer SectionDesign.id. Keep backward tolerance for the legacy hex id only if a flat-file fallback is still desired (optional; the parent issue's direction is SQLite-first).
4. Guard the "no iterations" / "no design" error paths that already exist (build.py:215-219).

Acceptance criteria

• Given an approved SectionDesign, POST /api/build/converse produces a topology and a saved build.
• Building an orchestrator-run section no longer requires any file under data/designs/.
• Missing/invalid section ids return a clean error event ({"type":"error", ...}), not a 500 traceback.

Testing

Local backend (cd backend && uvicorn app.main:app --reload --port 8080):

1. Run intake + a run until one section is approved (see parent issue Testing).
2. Note the section id from GET /api/runs/{id}.
3. POST /api/build/converse with that id; confirm it streams to build_saved.

Add a unit test that stubs the DB session and asserts converse_build reads from SectionDesign.record.

Out of scope

• Where the build record is saved (that's the BuildRecord sub-issue).
• Frontend wiring of a "Build this section" button (backend contract only here).