chore(governance): make the DCO checkbox mechanically satisfiable, or drop it

[lamemustafa]

Problem

The PR template's final checkbox asserts that commits include Signed-off-by: trailers. None of the 102 commits on the branch merged as #96 carried one, and no workflow enforces DCO — so nothing failed, and the gap surfaced only as a checkbox that had to be left unticked with an explanation.

It cannot be fixed retroactively. Adding trailers to already-pushed commits requires the rebase AGENTS.md forbids ("Never amend or rebase a pushed commit"). Once commits are pushed, the two rules leave no legal move, which is why this was recorded as an accepted gap in #96's disposition register rather than ticked.

Scope

Fix forward only — no existing history is rewritten.

First decide whether DCO is actually wanted for this project:

• If yes: add a commit-msg hook that appends the trailer when absent, so new commits satisfy it by construction, and enforce it in CI so the checkbox stops being an honour-system claim. Note that CI enforcement must apply from a chosen commit forward, or it will fail on all existing history.
• If no: remove the checkbox. A box that cannot be ticked truthfully trains everyone to tick boxes untruthfully, which is the more expensive failure — the template's value is that its other ~40 boxes are believed.

Acceptance criteria

• New commits carry a trailer without the author having to remember.
• The template checkbox is either mechanically satisfiable or gone.
• No pushed commit is amended or rebased.
• CONTRIBUTING.md states which of the two decisions was taken.