Drop the opencode serve wrapper when --output-schema lands upstream

[czpython]

What

The opencode harness starts opencode serve and POSTs one message, because the schema door (format: {type: "json_schema"}) exists only on the server API. The argv side lacks one flag.

Upstream state (verified 2026-09-02):

• opencode run builds the same Server.Default() app in-process (packages/opencode/src/cli/cmd/run.ts:948) and its prompt payload already accepts format — run.ts never passes it.
• feat(cli): add --output-schema flag anomalyco/opencode#17276 adds --output-schema to run. Open since 2026-03-13, no maintainer response, diff no longer applies.
• Their #25430 tracks retryCount being dead code.

What to do

1. Submit a fresh, rebased upstream PR with tests (the old diff was ~134 lines on run.ts).
2. When a release carries the flag: replace the serve-spawn/POST wrapper in backend/druks/harnesses/opencode.py with the flag on opencode run, keep the deadline and the typed-error parsing, delete the port-parse and abort plumbing.

Acceptance

• Upstream PR submitted and linked here.
• If merged: the harness invocation is argv-only and the suite passes unchanged.

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