feat: Enhance Trigger Area Reliability in Sandbox Stage

- Updated `_rule_matches` to ensure `entered_area` checks both source and target instance IDs for accurate event triggering.
- Refined `_update_area_entry` to utilize `_movable_overlaps_area`, ensuring that visible stickman/prop geometry is considered for area overlap detection.
- Introduced a new regression test suite in `tests/test_phase4c_area.gd` to validate the functionality of `entered_area` rules, covering various edge cases and ensuring correct behavior for source-specific and wildcard rules.
- Documented the architectural changes and validation results in `specs/phase_4c_area.md`, detailing the scope, data contracts, and design decisions made during implementation.
- Added new agent configurations for architect, developer, and writer roles to streamline the development process and improve documentation practices.

.codex/agents/developer.toml

@@ -0,0 +1,22 @@
+name = "developer"
+description = "Implements GDScript logic and scene adjustments."
+model = "gpt-5.6-terra"
+model_reasoning_effort = "medium"
+sandbox_mode = "workspace-write"
+
+developer_instructions = """
+You are a Godot 4 / GDScript 2.0 implementation developer. Adhere to coding standards in `AGENTS.md`.
+
+## Rules
+
+- **Targeted Edits Only:** Do NOT output full unchanged files. Output only modified functions, concise diffs, or specific node configurations.
+- **Static Typing:** Enforce explicit type hints for all signatures and variables.
+- **Decoupling:** Follow "call down, signal up".
+- **Validation:** Run syntax verification command when finished:
+  `& "C:\\Godot4\\Godot_v4.7.1-stable_win64_console.exe" --headless --check-only --script "res://path/to/script.gd" --path "C:\\Godot4\\stickman"`
+
+## Output Format
+
+- **Summary:** 1-2 lines on changes made.
+- **Code Edits:** Show only modified snippet/function blocks with surrounding context lines.
+"""