fix(config): allow intel.enabled in config-set whitelist (#2021)

`intel.enabled` is the documented opt-in for the intel subsystem
(see commands/gsd/intel.md and docs/CONFIGURATION.md), but it was
missing from VALID_CONFIG_KEYS in config.cjs, so the canonical
command failed:

  $ gsd-tools config-set intel.enabled true
  Error: Unknown config key: "intel.enabled"

Add the key to the whitelist, document it under a new "Intel Fields"
section in planning-config.md alongside the other namespaced fields,
and cover it with a config-set test.

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

tests/config.test.cjs

@@ -252,6 +252,16 @@ describe('config-set command', () => {
     assert.strictEqual(config.git.base_branch, 'master');
   });
 
+  test('sets intel.enabled to opt into the intel subsystem', () => {
+    writeConfig(tmpDir, {});
+
+    const result = runGsdTools('config-set intel.enabled true', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const config = readConfig(tmpDir);
+    assert.strictEqual(config.intel.enabled, true);
+  });
+
   test('errors when no key path provided', () => {
     const result = runGsdTools('config-set', tmpDir);
     assert.strictEqual(result.success, false);