Incrementally merge specific committed files from the agent's branch into main.

Allows a long-running agent to push a completed subset of its work (e.g. the first 5 of 10 test files) while still writing the rest. Reduces wall-clock time by making partial results available downstream earlier.

Only files that are already committed in the agent's worktree branch (agent/<session_id>) are merged. Uncommitted files are returned in uncommitted_files so the caller knows to commit them in the worktree first. Files that were already merged by a prior call are skipped and returned in skipped_already_merged.

Requires the task to be in_progress with a claimed_by_session set.