Add GPL-3.0 license, code signing policy, SignPath config

- GPL-3.0 LICENSE file (required for SignPath Foundation OSS program)
- CODE_SIGNING_POLICY.md (SignPath Foundation requirement)
- .signpath/config.yml (artifact signing configuration)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.signpath/config.yml

@@ -0,0 +1,15 @@
+# SignPath configuration for InstaDrums
+# This file defines the signing policy for the project
+
+version: v1
+
+artifact_configurations:
+  - name: windows-vst3
+    signing_policy: release-signing
+    artifact: InstaDrums-VST3-Win64.zip
+    description: "InstaDrums VST3 plugin for Windows x64"
+
+  - name: windows-standalone
+    signing_policy: release-signing
+    artifact: InstaDrums-Standalone-Win64.zip
+    description: "InstaDrums Standalone application for Windows x64"