fix(headless): record OpenCode prompt provenance accurately #1563

What happened

packages/headless/harbor/opencode_agent.py::_execution_identity() hashes MAKA_SYSTEM_PROMPT and records it as systemPromptHash.

The OpenCode adapter does not inject that prompt into OpenCode. _run_with_stop_sentinel() launches the pinned OpenCode CLI with the task instruction and OpenCode's benchmark config; OpenCode supplies its own native system scaffold. As a result, the recorded identity claims provenance for model-visible bytes that the OpenCode arm never received.

Changing only the Maka system prompt therefore changes the OpenCode cell's execution identity even though the OpenCode invocation is unchanged. This can create false cohort differences and misleading audit evidence. It does not change the OpenCode score or token usage itself.

Expected behavior:

• Execution identity distinguishes a Maka-injected prompt from a competitor-owned native prompt.
• An OpenCode cell does not record the Maka prompt hash as model-visible provenance.
• Changing only MAKA_SYSTEM_PROMPT does not change OpenCode's prompt identity.
• The pinned OpenCode version/toolchain remains the authority for the competitor-owned scaffold when its exact internal prompt hash is unavailable.

How to reproduce

1. Run two otherwise identical OpenCode harness cells.
2. Set MAKA_SYSTEM_PROMPT to different values in the two environments.
3. Compare maka-cell-execution-identity.json.
4. Observe different systemPromptHash values.
5. Compare the OpenCode launch command/config and provider-visible task input; the Maka prompt is not injected into either invocation.

Environment

• Maka: reproduced on main@213882bda and present in ee7e4ba5d
• Surface: Headless / Harbor OpenCode adapter
• OpenCode: pinned 1.17.18
• Node.js: 26.5.0
• Harbor: 0.13.2

Logs, screenshots, or additional context

This is a provenance defect, not an effectiveness or economy defect. Existing benchmark scores remain valid, but reports should not describe the recorded OpenCode systemPromptHash as the hash of OpenCode's actual native system prompt.

The fix should define the prompt-authority representation at the execution-identity contract rather than inventing a hash for inaccessible competitor-owned prompt bytes, and should include backward-compatible decoding for existing cell artifacts.