feat(client): game session management (#258, D-085)

Per-game save directories under user://saves/<timestamp>-<seed>/.
SessionManager autoload handles new_game(), resume_game(), quit flow.
Main menu scene with New Game / Continue / Quit buttons. Game-id
passed to server subprocess via --game-id flag.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/project.godot

@@ -11,7 +11,7 @@ config_version=5
 [application]
 
 config/name="The Settled Reach"
-run/main_scene="res://scenes/main.tscn"
+run/main_scene="res://scenes/main_menu.tscn"
 config/features=PackedStringArray("4.6", "GL Compatibility")
 config/icon="res://icon.svg"
 
@@ -23,6 +23,7 @@ InputMapper="*res://scripts/autoloads/input_mapper.gd"
 UIStrings="*res://scripts/autoloads/ui_strings.gd"
 FogState="*res://scripts/autoloads/fog_state.gd"
 AudioManager="*res://scripts/autoloads/audio_manager.gd"
+SessionManager="*res://scripts/autoloads/session_manager.gd"
 
 [audio]