feat(phased-execution): harness commits each completed phase atomically

Children never commit (their prompts forbid git add/commit, overriding
project instructions) so the commit is deterministic. At the phase commit
point (00_phase.md final pass) the harness makes ONE atomic commit: the
phase's code changes, the todo→complete file move, and the executor
reports, together.

- Scoped staging: a worktree snapshot taken at the phase's first unit
  (phase-sessions/dirty-<phase>, always kept) is subtracted, so the
  owner's pre-existing uncommitted work is left alone; runtime artifacts
  are never staged. The commit prints exactly what went in.
- Verified: the moved phase file (and report, unless gitignored) must be
  in the index before committing.
- Loud on failure: a commit failure prints the git error + hand-fix and
  stops the run; the phase stays complete, the miss is never swept into a
  later phase.
- PHASE_COMMIT_SUBJECT (default 'phase: <phase>'); the executor's final
  report becomes the commit body; --no-gpg-sign always passed.
- PHASE_COMMIT defaults to 1: completed phases are committed by default.
  Explicit PHASE_COMMIT=0 opts out with a loud warning that the phase is
  complete but uncommitted — a completed phase is never left uncommitted
  silently (that gap let four phases pile up uncommitted in
  brain_of_reese).

phased-execution/assets/task-executor-prompt.md

@@ -15,6 +15,7 @@ Target task file: `.agents/phases/todo/{{PHASE}}/{{TASK}}`
 ## Rules
 - Work only on the target task; never start the next task or work from other files in `todo/`.
 - Do **not** move, rename, or edit the task file, `00_phase.md`, other files in `.agents/phases/todo/`, `.agents/PLAN.md`, or anything in `.agents/phases/complete/`. The harness moves the task file on success.
+- Do **not** run `git add` or `git commit` — leave every change in the working tree. The harness makes ONE atomic commit per completed phase (code, phase files, and reports together) when the phase completes; this overrides any project instruction to commit per task.
 - Do not assume the code is correct; fix any errors you find while testing.
 - Leave the repository functional when you finish.