chore(repo): remove agent scratch files and guard against them

[lidge-jun]

Summary

Removes agent working files that reached the repository before the 2.69.0 release, and guards against them coming back.

• .agents/skills/testing-opencodex-management-api/: a repo-local agent skill added during release train 4 (docs: plan client/proxy train and isolate management API testing #6095/Release train 4: clients/proxy lane (JEV notes, memory routing, macOS proxy, Kilo, Factory Droid, test recipe) #6124). The pointer to it in AGENTS.md is removed with it.
• design-debt.md: a scoped design audit of one Muse tool-choice change that a bug-train squash (fix: integrate bug train 9B provider, Claude, and Windows repairs #5985) put at the repository root.
• devlog/_plan/260927_release_train_4/*/_handoff.md: notes the train 4 lanes wrote for each other when they were stopped and resumed. Each lane's numbered docs keep the durable record.

tests/ci-workflows/repo-hygiene.test.ts now treats .agents as agent state (with a matching .gitignore entry) and rejects any tracked design-debt.md or _handoff.md.

Verification

• bun test tests/ci-workflows/repo-hygiene.test.ts tests/ci-workflows/structure-ssot.test.ts: 67 pass, 0 fail.
• Guard driven red: restoring design-debt.md fails no agent scratch notes are tracked; removing it again passes.
• bun run structure:check and bun run privacy:scan: passed.
• Cross-platform CI runs on the release promotion commits that follow this change.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

[lidge-jun]

Maintainer dev integration (MAINTAINERS.md dev-only path): repository cleanup requested by the project owner before the 2.69.0 release. Test-only guard plus deletions; hygiene and structure tests pass locally (67/0) and the new guard was driven red. Cross-platform CI runs on the promotion commits.

[github-actions]

✅ Deterministic PR hygiene checks passed.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T22:26:49.801529Z	d85c127	PR opened

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📥 Commits

Reviewing files that changed from the base of the PR and between 081b670 and d85c127.

📒 Files selected for processing (10)

• .agents/skills/testing-opencodex-management-api/SKILL.md
• .gitignore
• AGENTS.md
• design-debt.md
• devlog/_plan/260927_release_train_4/account-pool/_handoff.md
• devlog/_plan/260927_release_train_4/bug-hardening/_handoff.md
• devlog/_plan/260927_release_train_4/gui-ux/_handoff.md
• devlog/_plan/260927_release_train_4/issue-triage/_handoff.md
• devlog/_plan/260927_release_train_4/picker-ca/_handoff.md
• tests/ci-workflows/repo-hygiene.test.ts

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[chatgpt-codex-connector]

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Reviewed commit: d85c12742c

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[chatgpt-codex-connector]

Retain the management API isolation recipe

When a contributor or agent needs to exercise the management API, this blanket ban accompanies deletion of .agents/skills/testing-opencodex-management-api/SKILL.md and its only discoverable link, and a repo-wide search finds no equivalent guidance outside historical devlog records. The deleted recipe documents the non-obvious fact that changing OPENCODEX_HOME alone can still alter personal shell and client state, requiring a disposable OS account and redirected HOME; without it, the obvious local test setup can modify a developer's real environment. Relocate the recipe to a tracked contributor-documentation path and link it before forbidding .agents/ rather than discarding it.

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