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/scripts/run-task.sh

@@ -40,7 +40,7 @@ build_pi_args
 if execute_unit "$unit"; then
   exit 0
 fi
-echo "✗ ERROR: task $unit FAILED after $MAX_FIX_ATTEMPTS attempts" >&2
-echo "  reports: $(unit_report_dir "$unit")/$(unit_base "$unit").a*.{md,err,validate}" >&2
-echo "  resume:  re-run this script — the failed executor session is resumed automatically" >&2
+# execute_unit printed the failure detail (task failure: errors, logs,
+# resume command — or phase-commit failure: the hand-fix command).
+echo "✗ ERROR: $unit did not complete — see the error output above" >&2
 exit 1