backend-agents B: spike — start/getRun/resumable AgentStreamEvent stream

[btipling]

Parent: #764 · Umbrella: #710
Label: backend-agents

Slice

Prove Vercel Workflows in this repo without cutting /api/agent over.

• Add Workflow SDK + Next withWorkflow on a preview-only path (do not change production turn owner in this slice).
• start(fixtureWorkflow) writes a handful of current AgentStreamEvents (text_delta, reasoning_delta, tool_start/tool_result, done) to getWritable().
• POST returns x-workflow-run-id and can pipe run.readable.
• GET /api/turns/:runId/stream?startIndex= uses getRun + getReadable({ startIndex }).
• Test: start, abort the HTTP client, poll getRun until completed; reconnect GET from startIndex=0 and from a mid index.

Non-goals

Real runAgent, sandbox, queue, HarnessHost cutover.

Cloud ops

Needs Workflows enabled on the Vercel project (slice D). Until then, local SDK test runtime only.

Refs: resumable streams · start

[btipling]

Slice B is done.

• Plan #787 IMPLEMENTED / closed.
• Implementation PR #788 merged 2026-08-23 (Closes #787; this source issue was not in the closing list).
• On main: app/api/turns/*, lib/workflows/turnsFixtureWorkflow.ts + turnsFixtureEvents.ts, AGENTS.md spike rows.

Source-issue checklist from this ticket is met: fixture streams current AgentStreamEvents via getWritable(), POST /api/turns returns x-workflow-run-id, GET …/stream?startIndex= resumes, abort≠cancel covered in the mocked-SDK suite. Non-goals (real runAgent, sandbox, queue, HarnessHost cutover) stay E/C/G as designed.

Closing to match sibling source issues #766 (C) and #767 (D). Live Workflows abort≠cancel against Production remains a residual for the smoke path / slice E — not a reason to keep this spike ticket open (#765 itself scoped that as local SDK test runtime until D).