Name the Abandon-Discard Exception in git-commit-conventions #3773

Found by the local review pass on the feature/burn-3761 pull request. That change applied the maintainer's answer on #3733, and the fix this issue asks for sits outside the files that change held.

Defect

backlog-burndown now gives a worker told to abandon its branch the developer's explicit instruction to run git reset --hard and git clean -fd in its own linked worktree. git-commit-conventions is the skill a worker actually loads when it is about to run those commands. It says they "require explicit developer instruction" and names one narrow cleanup exception, git branch -D after a squash merge. It does not name this one.

Failure

A worker that loads git-commit-conventions sees a ban with one listed exception, while the grant arrives in an orchestrator message. A worker that refuses fails safe, since the tree stays dirty and cleanup stops for the maintainer. But the two statements disagree, and the orchestrator ends up handing the maintainer exactly the cleanup #3733 set out to avoid.

Done When

GOVERNANCE.md "Git and Commit Rules" and the git-commit-conventions skill name the abandon-discard case alongside the post-squash git branch -D exception, or point to backlog-burndown for it.