backlog-burndown: four defects in the in-flight enumeration and claim traversal

[ptr727]

backlog-burndown "Grouping and File Claims" enumerates what is in flight before dispatching. Four defects in that enumeration.

• git fetch origin is specified without --prune, and this repository sets neither fetch.prune nor remote.origin.prune. A branch already deleted on the remote stays in git branch -r, then yields a real file set through git diff --name-only origin/develop...<branch>, so it collides forever. repo-worktree's own cleanup step says to fetch and prune.
• git diff --name-only origin/develop...<branch> hardcodes origin/develop where repo-worktree's base rule allows a main-based branch. On such a branch the three-dot merge base is wrong and the set over-reports everything develop carries and main does not.
• Every other input in that bullet comes with its command, and nothing says how to get from an enumerated branch name back to its claim comment across the issue backlog. The bullet that follows depends on that traversal ("walk from a worktree it found back to the claim explaining it") and supplies no mechanism either.
• "since the closing keyword is deliberately absent" is false. The mandated line is Closes on promotion: #N, which contains Closes. What actually makes it inert is that GitHub matches a keyword only immediately adjacent to its reference, and the unit never says so. As written a later reader "fixing" the apparent contradiction in either direction breaks the design.

Raised by the local-strict-review passes on the pull request that added this skill, and deferred there under that skill's own review-round budget.