Every commit MUST be signed off (git commit -s). Preserves the
copyright-holder's option to dual-license later — without a DCO/CLA
instrument, every external contribution erodes that option (ADR-0004).
Particularly load-bearing under multi-agent parallel authorship, where
multiple sessions commit in parallel and the DCO is the only per-commit
aggregation instrument.
Adds DCO.md with the full Linux-Foundation DCO text + the agents-sign-off-
with-the-human's-identity rule. AGENTS.md Git workflow gains the DCO bullet.
