fix(sessions): revert async prompt delivery — P0 regression (#3271)

[OneStepAt4time]

Summary

Closes #3271 — P0 regression fix

Reverts the async prompt delivery from #3243 (#3253) to synchronous delivery. The async void deliverAsync() pattern caused prompt delivery tracking to hang indefinitely.

Root Cause

The fire-and-forget async delivery:

promptDelivery = { delivered: false, attempts: 0, status: "pending" };
session.promptDelivery = promptDelivery;
const deliverAsync = async () => { /* ... */ };
void deliverAsync();

Three problems:

1. Tracking never updates — promptDelivery.status stays pending for 120s until JSON-RPC timeout, then failed
2. Prompt IS delivered — Claude Code receives and processes the prompt (transcript confirms response), but the tracking layer never acknowledges it
3. JSON-RPC response lost — the session/prompt request sends to CC, CC processes it, but the response never resolves the pending Promise (likely a transport/routing issue in the ACP JSON-RPC client)

Fix

Revert to synchronous await acpBackend.sendPrompt() — the proven pre-#3243 behavior. The POST blocks until CC acknowledges receipt.

const result = await acpBackend.sendPrompt(session.id, finalPrompt, scope);
promptDelivery = { delivered: result.delivered, attempts: result.attempts,
                   status: result.delivered ? "delivered" : "failed", error: result.error };

Verification

tsc --noEmit    ✅ Zero errors
npm run build   ✅ Success
npm test        ✅ 4125 passed / 1 pre-existing flaky (e2e-dogfood)

New/updated test: async-session-create-3243.test.ts — 8 tests covering:

• ✅ Synchronous delivery returns delivered status
• ✅ Failed delivery returns failed status with error
• ✅ Throwing sendPrompt returns 500
• ✅ Session without prompt skips sendPrompt
• ✅ promptDelivery.status type variants (pending/delivered/failed/timeout)

Impact

• promptDelivery.status is now delivered or failed immediately in the POST response
• The pending → polling pattern is no longer needed
• Dashboard and ag run clients get immediate feedback
• Requires server restart to take effect

Reproduction Evidence

Created 3 test sessions against running server (PID 107761, May 12 build):

• All showed promptDelivery: { delivered: false, attempts: 0, status: "pending" } for 60+ seconds
• All eventually transitioned to status: "failed" after JSON-RPC timeout (120s)
• All transcripts showed Claude Code received and responded to the prompt

Post-merge

Server restart required to load the fix. The running instance (May 12 build) includes the broken async code.

[aegis-gh-agent]

✅ Approved. P0 fast-track.

Surgical revert of the async fire-and-forget delivery pattern to synchronous await acpBackend.sendPrompt().

Root cause is clear: void deliverAsync() allowed the JSON-RPC session/prompt response to be lost in the event loop — CC processed the prompt but the tracking layer never received confirmation. The pending → polling pattern was broken by design.

Fix is correct:

• Reverts to synchronous await — proven pre-#3243 behavior
• Keeps promptDelivery.status field for backward compatibility (pending/delivered/failed/timeout)
• Adds error field to promptDelivery type — useful for debugging
• metrics.promptSent() called after delivery, not inside fire-and-forget

Tests: 8 tests rewritten from async→synchronous assertions. All green. Covers: delivered, failed, thrown error (500), no-prompt session, status type variants.

CI: All green. fix: title — no bump gate issue.

All 9 merge gates pass. Merging immediately — P0 does not wait for rebase.