fix(review): survive long reviews by bypassing undici bodyTimeout

[JohnnyVicious]

Summary

• Long adversarial reviews were dying at ~4.5 min with TypeError: terminated
• Root cause: sendPrompt used fetch(), which is bound to Node's bundled undici. Undici enforces a 300_000 ms default bodyTimeout and kills the socket the moment the OpenCode server holds the response body open for >5 min while the model thinks — which is the normal case for long reviews on slow/free models. Our outer AbortSignal.timeout(600_000) is a wall-clock abort, not a dispatcher-level body timer, so it cannot prevent that kill.
• Fix: replace the fetch() call with a direct node:http POST via a new httpPostJson helper. node:http has no equivalent hidden default, so the request survives until the server responds (or our own explicit wall-clock timer fires).
• Per-call budget is now 30 min, overridable via OPENCODE_COMPANION_PROMPT_TIMEOUT_MS, matching the tracked-jobs 30-min hard timer.

Repro / evidence

Reproduced the exact error locally:

caught after 275.7s
0: TypeError - "terminated" - undefined
1: BodyTimeoutError - "Body Timeout Error" - UND_ERR_BODY_TIMEOUT

The server sent headers + a single body byte, then stalled. Undici destroyed the socket at its 300 s default, and fetch() surfaced the cause as TypeError: terminated — byte-for-byte the message the failing review logged.

Test plan

• New regression test tests/send-prompt-body-timeout.test.mjs stalls the response body for 7 s and asserts the client tolerates it. Would have failed against the old fetch()-based implementation if its timeout were lowered below 7 s.
• Full suite: npm test → 222/222 pass.