FEATURE: allow runCommand to specify username or uid

[Pangjiping]

Summary

• Add user field to /command run payload and status response in specs/execd-api.yaml, using oneOf to accept either a POSIX username string or numeric UID; defaults to the execd process user when omitted.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• close FEATURE: support declaring uid and gid when running commands. #178
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[jwx0925]

@codex review

[chatgpt-codex-connector]

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