chore(privacy): remove personal absolute paths from public docs and plugin

[ManSio]

What

• De-personalize machine-specific absolute paths across human-facing docs and the opencode gate plugin.
• Untrack .local/ machine scratch scripts (added to .gitignore; files kept on disk).
• Add ests/test_no_personal_paths.py guard (scans git-tracked docs + plugin only).
• Pre-register experiments/4A_unit_of_return/ (unit-of-return 4-arm + agent-confound controls) — design only, no run.

Why

The public repo leaked the owner's local username in 49 tracked files and a drive-rooted project path in 115. No real secrets were found (only test fixtures such as AKIA…EXAMPLE). Git history is intentionally NOT rewritten (force-push on a public repo is out of scope).

Verification

• Guard: PASS.
• Full pytest: 1866 passed, 5 skipped, 0 failed.
• All pre-commit checks green.

Follow-up (not in this PR)

• F0c: ~48 live tracked files under experiments/, scripts/, tests/** still contain the username; edits there are paired with assertions / experiment provenance and need a reviewed pass.

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 9db6e8f4-6030-4456-bd36-1456ede570d1

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