feat: add --readpattern flag to print pattern contents to terminal

Adds a new `--readpattern <name>` CLI flag that prints the raw contents
of a named pattern's system.md file to stdout. This makes it easy to
inspect what instructions a pattern sends to the model without having
to navigate the filesystem manually.

The implementation respects custom patterns directories: it checks the
user's custom patterns directory first before falling back to the main
patterns directory, consistent with how all other pattern lookups work.