Re-cut a conflict in a split file mechanically instead of a full agent run (#6140, part 3) #6155

Split off from #6140, which landed the other three parts (rebases first in the check lane, idle builders take a busy builder's rebases, conflicted N, rebasing M, room R on the tick line and /flow).

What: when a pull's only conflicts are in a file that main split into per-entry files (like src/http/mcp.ts → src/http/mcp/tools/*, #5932), bring it up to date with a script, not a conflict agent run: take the pull's added or changed entry from the old file, write it to its per-entry file, regenerate the index, commit, push, and let the check run.

Done when:

• The finishing pass (src/crew/finishing.ts) and the conflict sweep (sweepConflicts in src/crew/autonomy.ts) recognise a conflict confined to a split file (the base deleted the file the pull changed, and the per-entry directory exists on the base) and try the re-cut before queuing an agent run.
• A re-cut that does not apply cleanly, or whose check is red, falls back to the agent run as today.
• The re-cut push counts as a rebase in the lane (src/checks/finishing_first.ts), and the run or log line records that it was mechanical.
• A test proves one pull conflicting only on the split file is brought up to date with no agent run, and one with another conflict still gets the agent run.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.