Add testing skill for the OpenCodex management API

[devin-ai-integration]

Adds a testing skill documenting how to boot OpenCodex as a foreground server on a scratch OPENCODEX_HOME and drive the Lab automation management API with curl: admin auth header, baseline/enable/disable state transitions, manual run dispatch, and clean shutdown. Verified against the real server on Windows.

Devin Session: https://app.devin.ai/sessions/cfc5342a3c4d4b7eb9c1b57b2adca7c6

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Review readiness checklist

This PR stays in draft until every box below is ticked. Tick all four boxes once the requirements are met:

• All CI tests are green on my local testing.

• I pushed my PR to the latest dev commit.

• I resolved all correct Codex and CodeRabbit findings.

• My PR is ready for review.

Open in Devin Desktop: https://app.devin.ai/desktop/session/cfc5342a3c4d4b7eb9c1b57b2adca7c6?variant=devin
Requested by: @luvs01

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✅ Deterministic PR hygiene checks passed.

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⏳ DRAFT

• review readiness checklist open (0/4 boxes ticked).

What to do

• Tick all four boxes in the PR description once you're done (currently 0/4).
• New commits were pushed after the checklist was completed on ef9ad96; the current head is ab64dac.
• The checklist has been reset: re-test against the latest code and tick all four boxes again.

Review readiness checklist

• ⬜ All CI tests are green on my local testing.
• ⬜ I pushed my PR to the latest dev commit.
• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

0/4 boxes ticked.

New commits were pushed after the checklist was completed on ef9ad96; the current head is ab64dac.
The checklist has been reset: re-test against the latest code and tick all four boxes again.
This PR stays in draft until every box above is ticked.