codeSplitter dispatch hint: warn when a new task's files are already being rewritten on an open branch #1163

Follow-up from #1155. Before a task starts, look at the files its issue or related PRs touch. If an open branch is already rewriting the same hot files, say so, so the queue can wait or stack the task on that branch (POST .../branches {name, parent}).

On 2026-10-12, four Two Fronts PRs all rewrote main.ts and rules.ts; bun scripts/merge-plan.ts shows #1078, #1087 and #1102 each blocking a later one on apps/two-fronts/src/main.ts.

The merge plan (src/merge-plan.ts) already has the per-file clash test; this needs a guess at a task's files before it has any edits, and a place in the dispatch (Agent Office queue or codeSplitter's agent runs) to show it.