new-sep scaffolding command

[pcarleton]

Tracks the scaffolding tool committed in SEP-2484 as a prerequisite for that SEP reaching Final.

v1: npx @modelcontextprotocol/conformance new-sep <number> creates sep-NNNN.yaml in the right place with sep:, spec_url: (derived from the SEP PR's changed spec files), and placeholder requirements showing the format:

sep: NNNN
spec_url: https://modelcontextprotocol.io/specification/draft/...
requirements:
  - text: "TODO: quote the normative sentence from the spec diff"
    check: sep-NNNN-todo
  - text: "TODO: requirement that cannot be tested"
    excluded: "TODO: reason"
    issue: https://github.com/modelcontextprotocol/conformance/issues/N

We probably want a corresponding Claude Code skill that wraps this — reads the SEP's spec diff, extracts net-new RFC 2119 requirements semantically, and fills the rows. Regex on added diff lines is insufficient (e.g. SEP-2164's -32602 bullet inherits its SHOULD from an unchanged parent line, so the keyword isn't in the diff).

To explore:

• Prompting for an existing scenario to extend rather than always creating a new one (per AGENTS.md "more checks, fewer scenarios")
• Detecting client vs server vs both from the requirement text

Related: reviewer guidance ("verify MUST/SHOULD against the spec diff, not the SEP prose" — see #200) should land as a section in AGENTS.md, since neither /review nor @claude review auto-discovers a standalone REVIEWING.md.