fix(cli): add allow_unicode=True and encoding="utf-8" to YAML I/O (#1936)

None of the yaml.dump() calls specify allow_unicode=True, causing
non-ASCII characters in extension descriptions to be escaped to
\uXXXX sequences in generated .agent.md frontmatter and config files.

Add allow_unicode=True to all 6 yaml.dump() call sites, and
encoding="utf-8" to all corresponding write_text() and read_text()
calls to ensure consistent UTF-8 handling across platforms.

src/specify_cli/agents.py

@@ -207,7 +207,7 @@ class CommandRegistrar:
         if not fm:
             return ""
 
-        yaml_str = yaml.dump(fm, default_flow_style=False, sort_keys=False)
+        yaml_str = yaml.dump(fm, default_flow_style=False, sort_keys=False, allow_unicode=True)
         return f"---\n{yaml_str}---\n"
 
     def _adjust_script_paths(self, frontmatter: dict) -> dict: