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.

capabilities/zcode/capability.json

@@ -0,0 +1,102 @@
+{
+  "id": "zcode",
+  "role": "runtime",
+  "version": "1.7.0-rc.2",
+  "title": "ZCode",
+  "description": "ZCode (Z.ai) — desktop Agentic Development Environment for GLM-5.2; Claude-shaped nested skills at ~/.zcode/skills/<name>/SKILL.md, slash commands, named subagents, native MCP; declarative plugin surface; profile-marker install; tier-2 community support.",
+  "tier": "core",
+  "requires": [],
+  "engines": {
+    "gsd": ">=1.6.0"
+  },
+  "runtime": {
+    "configHome": {
+      "kind": "dot-home",
+      "name": ".zcode",
+      "env": [
+        "ZCODE_CONFIG_DIR"
+      ]
+    },
+    "localConfigDir": ".zcode",
+    "configFormat": "none",
+    "artifactLayout": {
+      "global": [
+        {
+          "kind": "skills",
+          "destSubpath": "skills",
+          "prefix": "gsd-",
+          "nesting": "nested",
+          "recursive": false,
+          "converter": "convertClaudeCommandToClaudeSkill"
+        },
+        {
+          "kind": "commands",
+          "destSubpath": "commands",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": null
+        },
+        {
+          "kind": "agents",
+          "destSubpath": "agents",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": null
+        }
+      ],
+      "local": [
+        {
+          "kind": "skills",
+          "destSubpath": "skills",
+          "prefix": "gsd-",
+          "nesting": "nested",
+          "recursive": false,
+          "converter": "convertClaudeCommandToClaudeSkill"
+        },
+        {
+          "kind": "commands",
+          "destSubpath": "commands",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": null
+        },
+        {
+          "kind": "agents",
+          "destSubpath": "agents",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": null
+        }
+      ]
+    },
+    "commandStyle": "slash-hyphen",
+    "hooksSurface": "none",
+    "sandboxTier": "none",
+    "supportTier": 2,
+    "installSurface": "profile-marker-only",
+    "writesSharedSettings": false,
+    "permissionWriter": null,
+    "extendedHookEvents": [],
+    "hostIntegration": {
+      "embeddingMode": "declarative",
+      "commandSurface": "slash-file",
+      "dispatch": {
+        "namedDispatch": true,
+        "nested": "undocumented",
+        "maxDepth": "undocumented",
+        "background": false,
+        "subagentToolkit": "full",
+        "backgroundDispatch": false
+      },
+      "modelMode": "passive",
+      "hookBus": "host",
+      "stateIO": "filesystem",
+      "transport": "mcp",
+      "runtime": "electron"
+    }
+  }
+}