docs: invoke project skills with / in AI chat

[nishantmonu51]

• guide/ai/ai-chat.md: new "Using Project Skills" section on picking a skill with /, which skills each chat lists (analyst vs. developer), and the "Loaded skill" trace step, with a screenshot of the picker
• developers/build/ai-configuration.md#skills: how agents controls where a skill is listed, naming/description guidance for the picker, load-once vs. per-message behaviour, and always_apply
• guide/ai/mcp.md: skill tools are always served to users with AI access (Always serve the skill tools on the runtime MCP server and return runtime tool errors as tool results in admin/server/mcp.go #9934), list_skills returns all fields, and external clients ask for a skill by name

Documents #9932 and #9934.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!