feat: add new-sep scaffolding command

[pugsatoshi]

This pull request adds the new-sep <NNNN> CLI subcommand and a companion Claude Code skill that scaffolds a sep-NNNN.yaml requirement-traceability file for a new SEP — populating sep, spec_url (derived from the SEP's spec diff), and TODO requirements[] rows in the same style as src/scenarios/server/sep-2164.yaml.

Motivation and Context

Related #243.

SEP-2484 introduces the YAML format for requirement traceability. Currently, only sep-2164.yaml exists. Contributors must write the front matter from scratch, including the gh api ritual documented in AGENTS.md for extracting spec_url from docs/specification/draft/ diffs. This PR eliminates that manual work, allowing SEP-2484 to reach final status.

How Has This Been Tested?

Offline Scaffold from a Known Spec Path:

$ node dist/index.js new-sep 9999 --spec-path docs/specification/draft/server/resources.mdx
Wrote src/scenarios/server/sep-9999.yaml
Next steps:
  1. Edit the file to quote real normative sentences from the spec diff
     (and add a "#anchor" to spec_url if the requirement lives in a subsection)
  2. Implement the TypeScript scenario
  3. Register it in the appropriate suite list in src/scenarios/index.ts

$ cat src/scenarios/server/sep-9999.yaml
sep: 9999
spec_url: https://modelcontextprotocol.io/specification/draft/server/resources
requirements:
  - text: 'TODO: quote the normative sentence from the spec diff'
    check: sep-9999-todo
  - text: 'TODO: requirement that cannot be tested'
    excluded: 'TODO: reason'
    issue: https://github.com/modelcontextprotocol/conformance/issues/<NNNN>

Online lookup via GitHub API (modelcontextprotocol/modelcontextprotocol PR #2164):

$ node dist/index.js new-sep 9999 --pr 2164
Multiple spec files changed; picking docs/specification/draft/server/resources.mdx (most additions).
Resolved spec path: docs/specification/draft/server/resources.mdx
Wrote src/scenarios/server/sep-9999.yaml

The generated spec_url matches the page URL of the .mdx file that the PR changes. The output style is byte-identical to sep-2164.yaml.

--force guard verified (refuses to overwrite an existing yaml without the flag), --target override verified for client / server / authorization-server, and input validation (Invalid SEP number, Invalid --target, spec path must start with ...) verified.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context