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/adapter-imperative.test.cjs

@@ -25,7 +25,7 @@ const registry = require('../gsd-core/bin/lib/capability-registry.cjs');
 
 const RUNTIMES = Object.keys(registry.runtimes);
 
-test('imperative adapter: kind === "imperative" + runtime echoed + registry present, for all 15 runtimes', () => {
+test('imperative adapter: kind === "imperative" + runtime echoed + registry present, for every registry runtime', () => {
   for (const r of RUNTIMES) {
     const adapter = createImperativeAdapter({ runtime: r });
     assert.strictEqual(adapter.kind, 'imperative', `${r}: kind must be 'imperative'`);