fix(opencode): stop exporting parseCommandFile from plugin module (#301)

OpenCode's legacy plugin loader iterates every exported function in a
plugin module and tries to invoke each one with the plugin context object.
parseCommandFile was exported alongside the default plugin function, so
the loader called it with an object instead of a file path and crashed
with 'path must be a string or a file descriptor' before any hook ran,
which aborted the whole plugin load.

Move the frontmatter parser to its own .cjs sibling; the plugin module
now exposes exactly one top-level export (the default plugin function).
The plugin requires the parser via createRequire, and the existing
parseCommandFile tests now require the sibling directly.

Co-authored-by: ayushere <ayushere@users.noreply.github.com>