fix(windows): cross-platform path separators, JSON quoting, and dollar signs

- Add toPosixPath() helper to normalize output paths to forward slashes
- Use string concatenation for relative base paths instead of path.join()
- Apply toPosixPath() to all user-facing file paths in init.cjs output
- Use array-based execFileSync in test helpers to bypass shell quoting
  issues with JSON args and dollar signs on Windows cmd.exe

Fixes 7 test failures on Windows: frontmatter set/merge (3), init
path assertions (2), and state dollar-amount corruption (2).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/state.test.cjs

@@ -205,7 +205,7 @@ None
     );
 
     const result = runGsdTools(
-      "state add-decision --phase 11-01 --summary 'Benchmark prices moved from $0.50 to $2.00 to $5.00' --rationale 'track cost growth'",
+      ['state', 'add-decision', '--phase', '11-01', '--summary', 'Benchmark prices moved from $0.50 to $2.00 to $5.00', '--rationale', 'track cost growth'],
       tmpDir
     );
     assert.ok(result.success, `Command failed: ${result.error}`);
@@ -233,7 +233,7 @@ None
 `
     );
 
-    const result = runGsdTools("state add-blocker --text 'Waiting on vendor quote $1.00 before approval'", tmpDir);
+    const result = runGsdTools(['state', 'add-blocker', '--text', 'Waiting on vendor quote $1.00 before approval'], tmpDir);
     assert.ok(result.success, `Command failed: ${result.error}`);
 
     const state = fs.readFileSync(path.join(tmpDir, '.planning', 'STATE.md'), 'utf-8');