PresetManager.load already heals a forward-incompatible presets.json: the
block just above repairs the legacy `custom` shape and re-saves the file.
But if the file exists and is missing a whole built-in preset (e.g. an older
install written before `reason` existed), load returned it as-is, so that
built-in stayed permanently absent — silently missing from the picker that
GET /api/presets feeds, with no way for the user to get it back.
Extend the same self-heal: after the legacy migration, fill in any built-in
presets the loaded file is missing, defaults-first so user edits win, and
persist the result. This never clobbers an intentional removal — there is no
delete path for the built-in keys (only user_templates entries can be
deleted), and presets are hidden via an `enabled: False` flag, not removal.
Checks: python -m pytest tests/test_preset_fill_missing_defaults.py (3 passed;
2 fail on the pre-fix code), the existing preset cases in
tests/test_review_regressions.py still pass, python -m py_compile
src/preset_manager.py, git diff --check.
