docs: trim spec frontmatter to known fields (title only)

Official extension specs (ext-auth) use only `title:` in frontmatter
plus an <Info> protocol-revision marker. Remove the invented
extension_identifier / status / maintainers / related / profile_of
fields and restate the identifier and profile relationship in the body
prose where they belong.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

specification/draft/action-metadata.mdx

@@ -1,15 +1,11 @@
 ---
 title: Action Metadata
-extension_identifier: io.modelcontextprotocol/action-metadata
-status: Draft (experimental)
-maintainers:
-  - "@rreichel3"
-  - "@SamMorrowDrums"
-related:
-  - https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2061
-  - https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913
 ---
 
+<Info>**Protocol Revision**: draft</Info>
+
+**Extension identifier:** `io.modelcontextprotocol/action-metadata`
+
 > ⚠️ **Experimental draft skeleton.** This carries forward
 > [SEP-2061: Action Security Metadata](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2061)
 > by [@rreichel3](https://github.com/rreichel3) into the IG's experimental repo,