Add mutation testing with Stryker and refactor for testability

- Refactor codebase to separate pure logic from VS Code adapters
- Create src/core/ directory with framework-agnostic code
- Add Stryker mutation testing configuration
- Add 51 unit tests for core modules
- Achieve 63.70% mutation score (up from 5.48%)

Test improvements:
- typeHelpers: 0% → 62.50% (45 mutants killed)
- snippetGenerator: 0% → 73.33% (55 mutants killed)
- docblockGenerator: 0% → 62.79% (27 mutants killed)
- hookHelpers: 0% → 56.58% (43 mutants killed)
- matchers: 61.54% maintained (16 mutants killed)

All 208 existing tests pass with zero regressions.

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: johnbillion

.github/workflows/test.yml

@@ -33,3 +33,6 @@ jobs:
 
     - name: Run tests
       run: xvfb-run -a npm test
+
+    - name: Run mutation testing
+      run: npm run test:mutation

.gitignore

@@ -2,3 +2,5 @@ node_modules
 .vscode-test/
 *.vsix
 /out
+.stryker-tmp/
+reports/

.mocharc.unit.json

@@ -0,0 +1,6 @@
+{
+	"spec": "out/test/unit/**/*.test.js",
+	"ui": "tdd",
+	"timeout": 5000,
+	"color": true
+}