fix(#935): install changeset CLI so /gsd-update changelog preview works (#938)

- bin/install.js now copies scripts/changeset/ and scripts/lib/ into
  <configDir>/scripts/ so $GSD_DIR/scripts/changeset/cli.cjs resolves
  at runtime; aborts install with an explicit failure if the source
  directory is missing from the package.
- gsd-core/workflows/update.md: corrected path from
  gsd-core/scripts/changeset/cli.cjs to scripts/changeset/cli.cjs;
  added an explicit [ ! -f ] guard so a missing CLI surfaces a clear
  message rather than silently swallowing the error; stderr captured
  via 2>&1 sentinel so node errors are visible in the preview output.
- release.yml's changeset-CLI invocations (node scripts/changeset/cli.cjs)
  remain at the repo-root path and are unaffected by this change.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/changeset-cli.test.cjs

@@ -332,10 +332,13 @@ describe('changeset cli extract: version-range changelog extraction (#3496)', ()
   test('F1: workflows/update.md contains concrete extract subcommand invocation', (_t) => {
     const workflowPath = path.join(ROOT, 'gsd-core', 'workflows', 'update.md');
     const workflowText = fs.readFileSync(workflowPath, 'utf8');
-    // The invocation is: node "$GSD_DIR/gsd-core/scripts/changeset/cli.cjs" extract
-    // so the literal substring is 'cli.cjs" extract' (quote between script path and subcommand)
+    // The invocation uses either a direct path or an intermediate variable:
+    //   node "$GSD_DIR/scripts/changeset/cli.cjs" extract
+    //   node "$GSD_CHANGESET_CLI" extract
+    // Accept either form so future refactors don't immediately trip this anchor.
     assert.ok(
-      workflowText.includes('cli.cjs" extract') || workflowText.includes('cli.cjs extract'),
+      workflowText.includes('cli.cjs" extract') || workflowText.includes('cli.cjs extract') ||
+      (workflowText.includes('GSD_CHANGESET_CLI') && workflowText.includes('" extract')),
       'update.md must invoke cli.cjs extract (fix for #3496 BLOCKER 1)',
     );
     assert.ok(
@@ -351,6 +354,40 @@ describe('changeset cli extract: version-range changelog extraction (#3496)', ()
       'update.md must capture exit code or JSON output from extract',
     );
   });
+
+  // F2: update.md must use the INSTALLED path ($GSD_DIR/scripts/changeset/cli.cjs),
+  // NOT the old broken path ($GSD_DIR/gsd-core/scripts/changeset/cli.cjs).
+  // The installer copies scripts/changeset/ into <configDir>/scripts/changeset/,
+  // so the runtime path is $GSD_DIR/scripts/changeset/cli.cjs (#935).
+  // allow-test-rule: reads a product workflow .md file (not CJS source) to verify
+  // the runtime install path contract; there is no behavioural runtime to invoke.
+  test('F2: update.md CLI path is $GSD_DIR/scripts/changeset/cli.cjs (not gsd-core/scripts/…) (#935)', (_t) => {
+    const workflowPath = path.join(ROOT, 'gsd-core', 'workflows', 'update.md');
+    const workflowText = fs.readFileSync(workflowPath, 'utf8');
+    // The correct installed path must appear somewhere in the update workflow
+    assert.ok(
+      workflowText.includes('scripts/changeset/cli.cjs'),
+      'update.md must reference scripts/changeset/cli.cjs',
+    );
+    // The old broken path ($GSD_DIR/gsd-core/scripts/changeset/cli.cjs) must not appear
+    assert.ok(
+      !workflowText.includes('gsd-core/scripts/changeset/cli.cjs'),
+      'update.md must NOT reference the old gsd-core/scripts/changeset/cli.cjs path (fix for #935)',
+    );
+  });
+
+  // F3: update.md must guard against the CLI being missing (not pure silent-swallow)
+  // allow-test-rule: reads a product workflow .md file (not CJS source) to verify
+  // the guard is present; there is no behavioural runtime to invoke.
+  test('F3: update.md has an explicit guard when changeset CLI is missing (#935)', (_t) => {
+    const workflowPath = path.join(ROOT, 'gsd-core', 'workflows', 'update.md');
+    const workflowText = fs.readFileSync(workflowPath, 'utf8');
+    // The workflow must check for CLI existence before invoking it
+    assert.ok(
+      workflowText.includes('GSD_CHANGESET_CLI') && workflowText.includes('! -f'),
+      'update.md must guard against a missing changeset CLI with [ ! -f "$GSD_CHANGESET_CLI" ] (#935)',
+    );
+  });
 });
 
 describe('changeset cli render: file-I/O wrapper (#2975)', () => {