Add developers/.gdignore; let Godot 4 canonicalise saved scenes

The developers/aurelien sandbox scenes still reference legacy non-LOD
mesh paths (sm_book.mesh, sm_candlestick.mesh, ...) that don't exist
anymore — the production scenes were migrated to *_lod0/1/2.mesh.
Godot's full-project file scanner reads every .tscn at editor open
and emits 'Cannot open file' errors for each broken ext_resource,
even for scenes that the game never loads.

Mark developers/ as out-of-scope for Godot's filesystem walker with
a .gdignore at its root. The .import / .uid sidecars Godot had
generated for files inside (CheckLightmap.exr.import,
CheckLightmap.gd.uid) are auto-removed by the editor since they are
now orphaned metadata — only the source files (.tscn, .gd, .exr,
.lmbake) remain. Re-enabling the sandbox later just means deleting
the .gdignore; Godot will regenerate the sidecars.

The other diffs in this commit are Godot 4 canonicalising whatever
.tscn / .tres files were touched in the editor session that ran the
LightmapGI bake attempt: ext_resource uid= attributes added, format
bumped 2 -> 3, property order normalised. No behavioural change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scenes/levels/home/Home.tscn

@@ -1,6 +1,6 @@
-[gd_scene format=3]
+[gd_scene format=3 uid="uid://couv4chot3vuy"]
 
-[ext_resource type="PackedScene" path="res://scenes/levels/Levels.tscn" id="1"]
+[ext_resource type="PackedScene" uid="uid://c8cbqvsyyn3m3" path="res://scenes/levels/Levels.tscn" id="1"]
 
 [sub_resource type="Sky" id="1"]
 
@@ -11,7 +11,6 @@ background_energy_multiplier = 0.6
 sky = SubResource("1")
 ambient_light_color = Color(0.694118, 0.168627, 0.67451, 1)
 ambient_light_sky_contribution = 0.5
-ambient_light_energy = 1.0
 tonemap_mode = 2
 ssr_enabled = true
 glow_enabled = true
@@ -24,12 +23,9 @@ fog_depth_curve = 1.10957
 fog_depth_begin = 0.0
 fog_depth_end = 60.0
 
-[node name="Home" instance=ExtResource("1")]
+[node name="Home" unique_id=962070354 instance=ExtResource("1")]
 
-[node name="MainCamera" type="Camera3D" parent="." index="4"]
+[node name="MainCamera" type="Camera3D" parent="." index="4" unique_id=1199679200]
 transform = Transform3D(0.160708, -0.25632, 0.953138, 0, 0.96569, 0.259696, -0.987002, -0.0417351, 0.155194, 1.84336, 1.66169, 0.663199)
 environment = SubResource("2")
 fov = 38.5632
-__meta__ = {
-"_editor_description_": ""
-}