docs: add repo-naming rule (hyphens, not underscores)

New repos only, same non-retroactive caveat as trunk naming.
Underscores don't survive the trip to a domain, a Docker tag, or
most package-registry names, where hyphens do; matches the
kebab-case already used for branch descriptions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011EBCcjJvEvhpToahqdsLLA

git-operations.md

@@ -28,6 +28,9 @@ deviation from this default, and belongs in that project's own KB corpus, not he
 else), work with what's there. Never rename a repo's default branch as an incidental part of
 unrelated work.
 
+**Repo name: hyphens, not underscores** (`my-project`, not `my_project`) — for new repos, same
+caveat as trunk naming: don't rename an existing one over this.
+
 **Naming scheme: `type/short-description`.** A type prefix from the taxonomy below, then a
 kebab-case description — `feat/branch-naming-rules`, `fix/null-check-on-login`. No ticket ID in the
 name: a branch name should say what the change does, and if a tracker ticket exists, that reference