feat: discover Pi skills natively

A Pi pane listed exactly one hardcoded home directory. Discover Pi's own
skill roots instead: the skill directory of every Pi agent profile plus the
brand and generic directories, from the git root down to the cwd, with the
brand copy winning a name collision.

Drop Pi's entries from the resolver's default skill directories and default
command formats. A default format made the command config always report
"configured", which pinned Pi to the INI path and made native discovery
unreachable. Both maps now hold no defaults, so every provider discovers its own
directories and the INI remains a per-profile escape hatch.