docs: fix stale experimental-ext-skills links in ADR 0029 - #9486

Motivation & Context

ADR 0029's References section points at modelcontextprotocol/experimental-ext-skills, which has been renamed to modelcontextprotocol/ext-skills. Repository URLs redirect, but the two links that carry a path no longer resolve: the issue link returns 404, and the implementation-guidelines document has moved to docs/archive/sep-draft-skills-extension.md with a different heading.

Description & Review Guide

• What are the major changes? Three link updates in the References section of docs/decisions/0029-mcp-skill-templates-and-direct-references.md:
  • the implementation-guidelines link now points at ext-skills/blob/main/docs/archive/sep-draft-skills-extension.md#hosts-end-to-end-integration (the section is now titled "Hosts: End-to-End Integration");
  • the working-group link uses the current repository name;
  • the open-question link points at ext-skills/issues/39, which is the same issue.
• What is the impact of these changes? Documentation only; no code, tests, or behavior change.
• What do you want reviewers to focus on? That the three replacement URLs point at the intended content.

Checks:

• all three replacement URLs return HTTP 200;
• the hosts-end-to-end-integration heading exists in the archived document;
• the file is otherwise byte-identical to main — the diff is exactly 3 lines.

Related Issue

N/A - trivial change (documentation link fix)

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: link discovery, verification, and the edit were prepared with an AI assistant; I confirmed each replacement URL resolves and that the referenced heading exists, and reviewed the final diff.

Contribution Checklist

• The code builds clean without any errors or warnings — N/A: documentation only
• All unit tests pass, and I have added new tests where possible — N/A: documentation only
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change.