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.