Set up `ext-skills` repository with Skills extension specification docs

[tobi-oye]

Is your feature request related to a problem? Please describe.

SEP-2640 needed a proper home for the Skills extension specification and implementation-facing documentation.

Describe the solution you'd like

Set up the ext-skills repository with a stable specification page for SEP-2640, following the format used by other MCP extensions.

Primary implementation PR:

#138

Follow-up spec alignment:

#139

Additional context

This work was completed by @pja-ant and is being tracked here for Kanban completeness.