* Add get_commands RPC for headless clients
Headless clients like Emacs can now query which commands are available.
Previously they could only discover file-based prompt templates by
scanning the filesystem; extension commands and skills were invisible.
The response includes each command's name, description, and source
(extension, template, or skill). Commands appear in the same order
as the TUI's autocomplete: extension commands first, then templates,
then skills.
Built-in TUI commands (/settings, /fork, etc.) are excluded since
they require the interactive UI. Commands like /compact have dedicated
RPC equivalents instead.
* Add location and path to get_commands response
Clients can show where commands come from (user/project/path) and
display file paths in tooltips. The data is already available on
templates and skills - just exposing it.
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