feat: add ZCode (Z.AI) integration (#3063)

* feat: add ZCode (Z.AI) integration

Add a skills-based integration for ZCode, Z.AI's Claude-Code-style
agent. ZCode uses the same SKILL.md layout as Claude Code, so spec-kit
installs workflows into .zcode/skills/speckit-<name>/SKILL.md, invoked
in chat as $speckit-<name>.

- ZcodeIntegration(SkillsIntegration) with .zcode/ folder and --skills option
- Register in INTEGRATION_REGISTRY
- Catalog entry (tags: cli, skills, z-ai)
- Tests via SkillsIntegrationTests mixin
- Document in integrations reference and README

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: render $speckit-* invocations for ZCode skills

ZCode is documented as a skills agent invoked with $speckit-<command>,
but the central invocation rendering only special-cased codex, so
specify init Next Steps and extension hooks rendered the dotted
/speckit.<command> form instead.

Centralize the $speckit-* decision in a DOLLAR_SKILLS_AGENTS set with an
is_dollar_skills_agent() helper, and route both init Next Steps and
HookExecutor._render_hook_invocation through it. Add ZCode invocation
regression tests mirroring the existing Codex/Kimi coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

integrations/catalog.json

@@ -299,6 +299,15 @@
       "author": "spec-kit-core",
       "repository": "https://github.com/github/spec-kit",
       "tags": ["cli", "skills"]
+    },
+    "zcode": {
+      "id": "zcode",
+      "name": "ZCode",
+      "version": "1.0.0",
+      "description": "Z.AI ZCode CLI skills-based integration",
+      "author": "spec-kit-core",
+      "repository": "https://github.com/github/spec-kit",
+      "tags": ["cli", "skills", "z-ai"]
     }
   }
 }