feat: expand owner/repo shorthand for --install with a --git-host flag

--install someuser/repo expands to https://github.com/someuser/repo;
--git-host overrides the default host and forces source interpretation
even when the value matches an installed bundle name. Two or more path
segments are accepted so nested GitLab-style groups work, and #ref
pinning applies to shorthand values. --uninstall resolves owner/repo
against recorded sources: a single match uninstalls, multiple matches
prompt an interactive selection showing each bundle's source, and
non-interactive runs bail instead of guessing.

src/repl/mod.rs

@@ -876,8 +876,8 @@ pub async fn run_repl_command(
                     config::install_or_update_from_repl_args(value)?;
                 }
                 ReplInstallDispatch::Usage => println!(
-                    "Usage: .install <{}> | .install <git-url|installed-bundle> \
-                         [--filter <cat>] [--force]",
+                    "Usage: .install <{}> | .install <git-url|owner/repo|installed-bundle> \
+                         [--git-host <host>] [--filter <cat>] [--force]",
                     AssetCategory::NAMES.join("|")
                 ),
             },