[Doc] Point the Skills specification links at its beta directory

## Motivation and Context

SEP-3392 (Extension Lifecycle) is Final and marks the stage of an extension by the directory that holds its specification.
It classifies Skills as Beta, so ext-skills moved `specification/stable/skills.mdx` to `specification/beta/skills.mdx`
on 2026-10-29 (modelcontextprotocol/ext-skills#161) with the text unchanged. GitHub does not redirect a moved path,
so the link on the Skills docs page and the one in the `MCP::Skill` class comment answered 404.

Both now point at the beta path. The docs page also stops calling the text "stable", since that word now names
a lifecycle stage the extension has not reached.

## How Has This Been Tested?

The new URL answers HTTP 200 where the old one answers 404; the two files hold no other reference to the old path.

## Breaking Changes

None.

Author: koic

docs/_extensions/skills.md

@@ -10,8 +10,8 @@ Skills (SEP-2640) is a Final extension (negotiated via [Capability Extensions](/
 publish Agent Skills: folders holding a `SKILL.md` with YAML frontmatter plus supporting files, served as ordinary MCP resources
 under `skill://` URIs. The extension adds two methods, `skills/list` and `skills/get`, whose entries name a skill's `SKILL.md`,
 repeat its frontmatter, and list its files with SHA-256 digests and sizes, so a host can approve a skill from the listing and
-verify every file it later fetches. See the [Skills specification](https://github.com/modelcontextprotocol/ext-skills/blob/main/specification/stable/skills.mdx),
-the stable text behind SEP-2640.
+verify every file it later fetches. See the [Skills specification](https://github.com/modelcontextprotocol/ext-skills/blob/main/specification/beta/skills.mdx),
+the text behind SEP-2640, published at the Beta stage of the extension lifecycle (SEP-3392).
 
 ## Serving Skills
 

lib/mcp/skill.rb

@@ -26,7 +26,7 @@ module MCP
   # Instances are frozen value objects. Equality is identity: the client binds verified state to the exact
   # entry object it checked.
   #
-  # https://github.com/modelcontextprotocol/ext-skills/blob/main/specification/stable/skills.mdx
+  # https://github.com/modelcontextprotocol/ext-skills/blob/main/specification/beta/skills.mdx
   class Skill
     EXTENSION_ID = "io.modelcontextprotocol/skills"
     # SEP-2640 limits per skill: servers SHOULD NOT serve a skill beyond them, hosts MUST accept up to them.