v0.5.1 — clean self-update output

[0.5.1] - 2026-06-13

Patch. No breaking changes.

Changed

• self-update output is now a short progress summary instead of git's raw
  fetch/merge log. It prints checking for updates…, applying N new commit(s)…, and updated to dctl <version> — no object counting, no
  From github.com… line, no fast-forward diffstat, and none of install.sh's
  routine output. The installer's output is captured and shown only if it
  fails. already up to date stays a one-liner with the version.