Presets: fill missing built-in defaults on load

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.

src/preset_manager.py

@@ -92,6 +92,18 @@ Use precise language. Show causal relationships explicitly. Quantify uncertainty
                     custom.setdefault("inject_prefix", "")
                     custom.setdefault("inject_suffix", "")
                     self.save(presets)
+            # Heal a forward-incompatible file the same way the legacy `custom`
+            # migration above does: fill in any built-in presets an older or
+            # partial presets.json is missing, so they reach existing installs
+            # (a missing built-in is otherwise silently absent from the picker
+            # served by GET /api/presets). There is no delete path for the
+            # built-in keys, so this never clobbers an intentional removal.
+            # Defaults first, loaded values win — user edits are preserved.
+            if isinstance(presets, dict) and any(
+                k not in presets for k in self.DEFAULT_PRESETS
+            ):
+                presets = {**self.DEFAULT_PRESETS, **presets}
+                self.save(presets)
             return presets
         except Exception as e:
             logger.error(f"Error loading presets: {e}")