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.

src/runtime-name-policy.cts

@@ -196,6 +196,7 @@ const RUNTIME_LABELS: Readonly<Record<string, string>> = {
   kimi: 'Kimi CLI',
   codebuddy: 'CodeBuddy',
   cline: 'Cline',
+  zcode: 'ZCode',
 };
 
 /**
@@ -266,7 +267,7 @@ export function getGlobalConfigHomeFragment(runtime: string): string {
  */
 const RUNTIME_FLAG_IDS = Object.freeze([
   'opencode', 'kilo', 'codex', 'copilot', 'antigravity', 'cursor',
-  'windsurf', 'augment', 'trae', 'qwen', 'hermes', 'codebuddy', 'cline', 'kimi',
+  'windsurf', 'augment', 'trae', 'qwen', 'hermes', 'codebuddy', 'cline', 'kimi', 'zcode',
 ] as const);
 
 /**