ci: add DCO enforcement

[gchahal1982]

Summary

• add the standard Agent Studio Open DCO workflow with the required DCO / dco job name
• add CONTRIBUTING guidance for git commit -s and CODEOWNERS-sensitive changes

Verification

• git diff --check HEAD~1..HEAD
• commit includes a Signed-off-by trailer

This moves the Open Studio DCO launch gate forward for the Agent public repo. After merge, branch protection can require DCO / dco like Rubric and Robotics.

[gchahal1982]

2026-05-20 review-ready status:

• PR is mergeable and auto-merge is armed.
• DCO / dco passed at head 081812ea19d2c84bf50be133031accd0d4b77424.
• This adds the missing Agent DCO workflow and contributor guidance so Agent can match Rubric/Robotics DCO enforcement after merge.

Remaining action for this PR: required human/CODEOWNERS approvals from Agent Maintainers and Security, then auto-merge can complete and Agent branch protection can require DCO / dco.