Normalize bare owner/repo npm sources and detect installed bin

Fixes a confusing failure when users pass --npm owner/repo: npm treats it
as a GitHub shorthand instead of a scoped package, so no binary is
created and the install fails with "binary not found". NewNpmInstaller
now rewrites bare owner/repo strings to @owner/repo, and Install snapshots
the npm global bin dir to discover binaries with unexpected names and
report a helpful error when none are produced.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Reinstall script plugins on init when the install URL changes (#15)

Add the auth instruction for skills

Update the source repo before installing skills

Allow using the interactive UI to install plugins

Warning if removes a non-exists plugin

Only download the missing plugins in command

Change the skill subcommand to install skill subcommand for CLI

Add skill subcommand for CLI

Fix brew update output