Add prop interaction system and throw mechanics specification

- Introduced `stickman_prop_interactor.gd` to manage prop interactions.
- Created `phase_11_prop_interactions.md` detailing the design and implementation of prop pickup, drop, and throw mechanics.
- Defined interaction states, action lifecycles, and event triggers for prop handling.
- Specified constraints for throw actions, including target positioning and velocity calculations.
- Established acceptance criteria for prop interactions and lifecycle management.

props/briefcase.prop

@@ -11,7 +11,7 @@
 		"primary_grip": {
 			"allowed_hands": "both",
 			"role": "grip",
-			"rotation_deg": 0.0,
+			"rotation_deg": 90.0,
 			"x": 0.0,
 			"y": -82.066291809082,
 			"z_index_offset": 1
@@ -19,8 +19,8 @@
 	},
 	"material_preset": "wood",
 	"metadata": {
-		"created_at": "2026-09-22T00:22:32",
-		"modified_at": "2026-09-22T00:22:32"
+		"created_at": "2026-09-24T23:17:51",
+		"modified_at": "2026-09-24T23:17:51"
 	},
 	"prop_name": "Brown Briefcase",
 	"shapes": [