repo descriptions (native git description file) !1

Implements #4 using the native git description file rather than a database column: no migration, and descriptions ride along in backups, transfers, and org renames for free. The git-init placeholder is treated as empty.

• repo create --description and repo settings description (repo admin; first line, 256-char cap)
• repo show / repo list include it, human and --json
• web: description under the repo header, columns on index and owner pages
• forks inherit the source description
• e2e: TestRepoDescriptions covers create/update/denial, placeholder suppression, fork inheritance, and all three web surfaces

approval was stale at time of merge simply due to rebasing back onto main; approval is still valid