feat(#1925): add ZCode (Z.ai) as a pluggable runtime descriptor

Add ZCode as a first-party runtime via a declarative capability descriptor
(capabilities/zcode/capability.json) with zero hardcoded runtime === 'zcode'
branches — exercising the de-hardcoded, data-driven runtime path that 1.7.0
(ADR-1016 / ADR-1239) enables.

Descriptor (all axes sourced verbatim from zcode.z.ai docs):
- configHome ~/.zcode; nested skills + flat commands/agents; profile-marker install
- Claude-shaped skill format reuses convertClaudeCommandToClaudeSkill (no new converter)
- hostIntegration: declarative / slash-file / mcp / electron; dispatch background=false
  (foreground-only per docs); nested+maxDepth undocumented; passive model mode

Installer registration (data, not branches): --zcode flag, allRuntimes, runtimeMap,
interactive menu, --all list. getGlobalConfigDir + resolveRuntimeArtifactLayout +
ALLOWED_CONFIG_RUNTIMES + resolveInstallPlan all derive from the descriptor.

Revamped the brittle per-runtime golden-master tests to be count-agnostic,
descriptor-derived property tests (1.7.0 makes runtimes pluggable data, so pinning
frozen '15 runtime' snapshots is the wrong invariant): getdirname, label-policy,
config-adapter-registry (intent + install-plan golden master), capability-registry,
host-integration-descriptors (counts derive from curated maps). Adding a runtime
descriptor now extends coverage with zero edits to those suites.

Welcome banner, --zcode help, supported-runtimes how-to, and the host-integration
capability matrix (every axis cited) updated.

tests/runtime-flags.test.cjs

@@ -14,7 +14,7 @@ const registry = require('../gsd-core/bin/lib/capability-registry.cjs');
 const EXPECTED_FLAGS = [
   'isOpencode', 'isKilo', 'isCodex', 'isCopilot', 'isAntigravity',
   'isCursor', 'isWindsurf', 'isAugment', 'isTrae', 'isQwen', 'isHermes',
-  'isCodebuddy', 'isCline', 'isKimi',
+  'isCodebuddy', 'isCline', 'isKimi', 'isZcode',
 ];
 
 test('runtimeFlags: every known non-claude runtime sets exactly its own flag true', () => {
@@ -35,12 +35,12 @@ test('runtimeFlags: claude / unknown / empty → all flags false (fail-closed)',
   }
 });
 
-test('runtimeFlags: all 14 flags present + boolean + the object is frozen', () => {
+test('runtimeFlags: all 15 flags present + boolean + the object is frozen', () => {
   const flags = runtimeFlags('opencode');
   for (const f of EXPECTED_FLAGS) {
     assert.strictEqual(typeof flags[f], 'boolean', `${f} must be boolean`);
   }
-  assert.deepStrictEqual(Object.keys(flags).sort(), [...EXPECTED_FLAGS].sort(), 'exactly the 14 flags');
+  assert.deepStrictEqual(Object.keys(flags).sort(), [...EXPECTED_FLAGS].sort(), 'exactly the 15 flags');
   assert.ok(Object.isFrozen(flags), 'flags object must be frozen');
 });