feat: add ZCode (Z.AI) integration

[meymchen]

Closes #2975

Summary

Adds a skills-based integration for ZCode (Z.AI), a Claude-Code-style coding agent. ZCode uses the same SKILL.md skill layout as Claude Code, and Z.AI's own docs recommend Skills (over simple / commands) for template- and script-driven workflows — which is exactly what spec-kit is.

Spec-kit workflows install into .zcode/skills/speckit-<name>/SKILL.md and are invoked in chat as $speckit-<name>.

Changes

• src/specify_cli/integrations/zcode/__init__.py — new ZcodeIntegration(SkillsIntegration): folder .zcode/, skills dir .zcode/skills, context_file="ZCODE.md", requires_cli=True (key zcode matches the CLI executable), and a --skills option.
• src/specify_cli/integrations/__init__.py — registered import + _register(ZcodeIntegration()) (alphabetical).
• integrations/catalog.json — catalog entry, tags ["cli", "skills", "z-ai"].
• tests/integrations/test_integration_zcode.py — SkillsIntegrationTests mixin (29 tests).
• docs/reference/integrations.md + README.md — documentation.

Modeled directly on the existing Claude/Kimi skills integrations. Two low-risk defaults: context file ZCODE.md (consistent with the per-tool pattern; zcode docs don't pin one down) and minimal skill frontmatter (name, description) per zcode's documented format.

Testing

• uv run pytest tests/integrations/test_integration_zcode.py → 29 passed
• Full integration suite passes; the single failure in tests/test_extensions.py::TestCatalogStack::test_default_stack is pre-existing on main and unrelated to this change.

Manual test results

Agent: ZCode (via specify init) | OS/Shell: Linux/bash

Command tested	Notes
specify init --integration zcode	Scaffolds all 11 skills under .zcode/skills/speckit-*/SKILL.md with correct name/description frontmatter matching zcode's documented format.

AI assistance disclosure

This contribution was developed with AI assistance. The integration was researched against the zcode docs, implemented by mirroring existing skills-based integrations, and verified by me with the automated tests and the specify init smoke test above.