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/helpers/install-shared.cjs

@@ -56,6 +56,7 @@ const RUNTIME_META = {
   qwen:         { localDir: '.qwen',             globalSuffix: '.qwen' },
   trae:         { localDir: '.trae',             globalSuffix: '.trae' },
   windsurf:     { localDir: '.windsurf',          globalSuffix: path.join('.codeium', 'windsurf') },
+  zcode:        { localDir: '.zcode',             globalSuffix: '.zcode' },
 };
 
 // Runtimes that emit per-skill files under skills/ (not rules-based or commands-based)