feat(cover-note): add kind-1624 cover notes for PRs, patches, and issues

Implements experimental kind-1624 cover note events:

- KIND_COVER_NOTE constant and process_cover_note() in git_events.rs;
  replaceable semantics (latest created_at, hex-id tiebreak), author or
  maintainer only
- kind-1624 events fetched alongside labels in the fetch pipeline;
  cover_notes count added to FetchReport display
- ngit pr/issue view: cover note displayed in place of description with
  a clear 'Cover Note:' header; maintainer-authored notes identify the
  author; original description shown only with --comments; cover_note
  object included in --json output
- ngit pr set-cover-note / ngit issue set-cover-note: publish a
  kind-1624 event; nostr: mentions in --body converted to q/p tags via
  tags_from_content (same rules as issue --body)
- Fix pre-existing clippy::too_many_lines on repo/mod.rs show_info

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