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.

docs/how-to/install-on-your-runtime.md

@@ -408,6 +408,22 @@ Skills land in `~/.trae/`. GSD installs skills, agents, and rule references.
 
 ---
 
+### ZCode
+
+```bash
+npx @opengsd/gsd-core@latest --zcode --global
+```
+
+[ZCode](https://zcode.z.ai/en) is Z.ai's desktop Agentic Development Environment for the GLM-5.2 model. GSD installs skills (nested `SKILL.md` bundles), slash commands, and subagents under `~/.zcode/`:
+
+- **Skills** → `~/.zcode/skills/gsd-<name>/SKILL.md` (invoke with `$gsd-<name>` in chat)
+- **Commands** → `~/.zcode/commands/gsd-<name>.md` (invoke with `/gsd-<name>`)
+- **Subagents** → `~/.zcode/agents/gsd-<name>.md`
+
+ZCode's skill format is identical to Claude Code's, so no runtime-specific converter is required — GSD lands as a pure declarative descriptor with no hardcoded installer branches. ZCode also natively imports skills and MCP config from `~/.claude`; if you install GSD for **both** Claude and ZCode, you may see duplicate GSD skills inside ZCode, which is expected. To connect ZCode's MCP servers to GSD's companion server, see [how to connect the GSD MCP server](connect-gsd-mcp-server.md).
+
+---
+
 ## Local vs global install
 
 All examples above use `--global`, which installs GSD once for your user account. To scope an install to a single project, replace `--global` with `--local`: