plans(v3-ergonomics): close out one-way-to-spell sweep (#604)

* plans(v3-ergonomics): close out one-way-to-spell sweep

§7 first bullet flipped to [x] — 4 implementation PRs shipped (#600-#603).
Migration guide §20/§21/§22 already document the path moves; PR 4 was
non-breaking (docs.rs nav only). Plan file pruned per its own close-out.

* plans(v3-ergonomics): tighten §7 close-out entry (/simplify)

- Per-PR sub-bullets instead of dense paragraph: future readers scan
  vertically and grep by PR number.
- Drop "Zero external callers ... (rule 23 N/A)" — PR-process narration;
  numbered rule reference would rot.
- Drop "plan ... was pruned" sentence — reader is in the tracker; the
  deleted sibling file is invisible to them.