fix(skills): preserve non-ASCII characters in skill frontmatter (#2917)

* fix(skills): preserve non-ASCII chars in skill frontmatter

Skill SKILL.md frontmatter descriptions containing non-ASCII
characters were escaped to \uXXXX / \xXX sequences because
yaml.safe_dump() was called without allow_unicode=True.

- Add allow_unicode=True to the 7 skill/command frontmatter
  safe_dump sites (extensions, presets, claude integration)
- Add regression tests for the render and extension-install paths

Follows the approach of #1936; encoding="utf-8" is already set on
the affected write paths, so no encoding change is needed here.

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

* refactor(_utils): add dump_frontmatter helper

Centralize skill/command frontmatter YAML serialization into a single
_utils.dump_frontmatter helper so no call site can drop allow_unicode or
diverge on formatting. Route the 7 existing sites through it and drop a
now-unused local yaml import.

Switch the extension test fixtures to yaml.safe_dump for parity with the
production safe-dump/safe-load codepaths.

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

---------

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

src/specify_cli/integrations/claude/__init__.py

@@ -5,10 +5,9 @@ from __future__ import annotations
 from pathlib import Path
 from typing import Any
 
-import yaml
-
 from ..base import SkillsIntegration
 from ..manifest import IntegrationManifest
+from ..._utils import dump_frontmatter
 
 # Mapping of command template stem → argument-hint text shown inline
 # when a user invokes the slash command in Claude Code.
@@ -103,7 +102,7 @@ class ClaudeIntegration(SkillsIntegration):
         skill_frontmatter = self._build_skill_fm(
             skill_name, description, f"templates/commands/{template_name}.md"
         )
-        frontmatter_text = yaml.safe_dump(skill_frontmatter, sort_keys=False).strip()
+        frontmatter_text = dump_frontmatter(skill_frontmatter)
         return f"---\n{frontmatter_text}\n---\n\n{body.strip()}\n"
 
     def _build_skill_fm(self, name: str, description: str, source: str) -> dict: