feat(skills): serve the Orbit skill for the caller query mode

What does this MR do and why?

Users with GQL queries turned on get the JSON-oriented Orbit skill today. This serves a GQL version of the skill to those users, with its own syntax reference and paste-ready queries, and keeps the JSON version for everyone else.

Related Issues

Relates to #1311

Rails still has to pass the user's query language on skill requests. Until it does, every caller gets the JSON skill.

Testing

local gdk

Performance Analysis

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.

Agent context

• ListSkills and GetSkill take language. The default is JSON; an unknown value returns INVALID_ARGUMENT.
• GQL callers get skills/orbit/SKILL.gql.md as SKILL.md. Its version is 0.33.0+gql, so ETags and CLI caches don't mix modes.
• references/gql.md goes only to GQL callers, and the JSON query references and remote repo-map helper go only to JSON callers. Every other file is shared.
• The docs smoke test now runs gql orbit-query fences.