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.

.changeset/zcode-runtime-1925.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 0
+---
+**ZCode (Z.ai) is now an installable runtime** — a desktop Agentic Development Environment for the GLM-5.2 model can now be targeted with `--zcode`, landing GSD skills at `~/.zcode/skills/<name>/SKILL.md` plus slash commands and subagents. ZCode ships as a pure declarative capability descriptor (`capabilities/zcode/capability.json`) with zero hardcoded `runtime === 'zcode'` branches, reusing the Claude skill converter — the de-hardcoded, data-driven runtime path that 1.7.0 (ADR-1016 / ADR-1239) enables. (#1925)