Help discoverability improved; web catalog surfaced

Minor Changes

• a3a08d5: Improve top-level help discoverability and surface the web catalog.
  • releases --help (and releases help) now list every command — lookup, collection, auth, whoami, completion, skills, and more — instead of the curated seven. Bare releases still shows the friendly quick-start.
  • The landing screen now shows the web catalog (https://releases.sh) in its header, and the full --help listing carries it too.
  • Removed the Exit codes: see README.md#exit-codes footer line from the landing screen.
  • The curated landing now labels the command tail (matching the "most common commands" block) instead of its latest alias.
  • The first-run completion hint no longer nags when completions are already installed by the package manager (e.g. Homebrew generates them at install time). Documented that Homebrew installs completions automatically while npm / shell-installer / binary users run releases completion install once.
  • The landing screen and --help now show a persistent, self-resolving notice when shell completion isn't set up (e.g. for npm / binary installs) — Shell completion isn't set up — run "releases completion install <shell>". It disappears automatically once completions are detected (user- or package-manager-installed), is TTY-gated so it never pollutes piped output, and respects RELEASES_NO_COMPLETION_HINT.
  • Updated the product tagline from "Changelog indexer and registry for AI agents and developers" to "The changelog & release-notes registry for developers and AI agents" across the CLI landing, --help, package descriptions, the Homebrew formula, and the README.