v1.15.1

Fixes a class of confusing failures when installing agents or skills from a
URL, and clarifies the install/register forms.

Fixed

• smith {agent,skill} install --from <url> (and the GUI "Install from URL"
  discover flow) no longer crash with the generic "This is a bug in
  agent-smith" message on foreseeable bad input. Bad/unreachable URLs, missing
  branches, malformed URLs, and clone-directory collisions now produce a
  friendly, actionable error (network / not-found / usage) across every agent
  and skill install/discover/sync path.
• GitHub "web" URLs like https://github.com/owner/repo/tree/<branch>/<subpath>
  (what you get by copying the browser address bar) now work: smith strips the
  /tree|blob/<branch>/<subpath> suffix to the cloneable repo URL and uses the
  branch as the ref when one isn't supplied.

Changed

• GUI: install/register/create inputs now show concrete example placeholders
  (e.g. https://github.com/owner/repo, ~/my-agents, bundle.tgz) and an
  info (?) tooltip explaining what each field accepts.
• GUI: the "register catalog" form now detects a pasted URL/archive in its
  local-path field and points you at "Install from URL" instead of the
  confusing "not a git repo" message.