feat: add local quality gate scripts and documentation

Add quality.sh (bash), quality.ps1 (PowerShell), and fix_format.sh
for automated GDScript quality checking. Includes:
- gdformat --check (formatting gate)
- gdlint (lint rules)
- godot --headless project validation (with 30s timeout)
- Optional GUT test support
- --changed mode for checking only modified .gd files
- Compact summary output with NEXT FIX suggestions
- Full logs saved to logs/quality/latest/

Add docs/local_quality_gate.md with setup and usage instructions.

docs/local_quality_gate.md

@@ -0,0 +1,106 @@
+# Local Quality Gate
+
+A single command to detect broken GDScript, formatting issues, lint problems,
+parser errors, and LLM-generated garbage before committing.
+
+## Requirements
+
+- **Python 3** with `gdtoolkit` (gdscript-toolkit):
+  ```bash
+  pip install gdtoolkit
+  ```
+- **Godot 4** binary on `PATH`, or set the `GODOT_BIN` environment variable
+
+## Usage
+
+### Full project check
+
+```bash
+# Linux / macOS / Git Bash
+./tools/quality.sh
+
+# Windows PowerShell
+pwsh ./tools/quality.ps1
+```
+
+### Fast changed-files mode
+
+Only runs `gdformat` / `gdlint` on `.gd` files modified since the last commit.
+The Godot parser check still runs on the whole project.
+
+```bash
+./tools/quality.sh --changed
+pwsh ./tools/quality.ps1 -Changed
+```
+
+### Auto-format (not part of the gate)
+
+```bash
+./tools/fix_format.sh
+```
+
+## What it checks
+
+| Check | Tool | What it detects |
+|---|---|---|
+| **gdformat** | `gdformat --check .` | Unformatted code (indentation, spacing) |
+| **gdlint** | `gdlint .` | Lint violations (unused args, long lines, naming, etc.) |
+| **godot-check** | `godot --headless --check-only` | Parser/syntax errors, missing dependencies |
+| **gut** | `godot -s gut_cmdln.gd` | Failing GUT unit tests (only if addon exists) |
+
+## Output
+
+If everything passes:
+
+```
+QUALITY RESULT: PASS
+
+  gdformat     PASS
+  gdlint       PASS
+  godot-check  PASS
+  gut          SKIPPED
+
+Full logs:
+  logs/quality/latest/
+```
+
+On failure, only relevant errors and fix suggestions are shown:
+
+```
+QUALITY RESULT: FAIL
+
+  gdformat     FAIL
+  gdlint       FAIL
+  godot-check  PASS
+  gut          SKIPPED
+
+    res://simulation/SimNPC.gd:42 Unused argument 'delta'
+    res://simulation/task_manager.gd:88 Line too long (120 > 100)
+
+  NEXT FIX:
+    1. Fix unused-argument in SimNPC.gd
+    2. Run gdformat to auto-format files
+
+Full logs:
+  logs/quality/latest/
+```
+
+Full logs are always written to `logs/quality/latest/` and never dumped
+to the terminal.
+
+## Environment variables
+
+| Variable | Description |
+|---|---|
+| `GODOT_BIN` | Path to the Godot executable (auto-detected from PATH if unset) |
+
+## CI setup
+
+To add this to CI, install the dependencies and run:
+
+```bash
+pip install gdtoolkit
+./tools/quality.sh
+```
+
+The script exits non-zero on any failure, so it will fail the CI step.