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/config/request_context.rs

@@ -3500,11 +3500,16 @@ impl RequestContext {
                 values = super::map_completion_values(
                     InstallFilter::NAMES.iter().map(|s| s.to_string()).collect(),
                 );
+            } else if prev == "--git-host" {
+                values = super::map_completion_values(vec!["github.com".to_string()]);
             } else {
                 let has_filter = args.iter().enumerate().any(|(i, a)| {
                     a.starts_with("--filter=") || (*a == "--filter" && i < args.len() - 1)
                 });
                 let has_force = args.contains(&"--force");
+                let has_git_host = args.iter().enumerate().any(|(i, a)| {
+                    a.starts_with("--git-host=") || (*a == "--git-host" && i < args.len() - 1)
+                });
                 let mut available: Vec<&str> = vec![];
 
                 if !has_filter {
@@ -3513,6 +3518,9 @@ impl RequestContext {
                 if !has_force {
                     available.push("--force");
                 }
+                if !has_git_host {
+                    available.push("--git-host");
+                }
 
                 values = super::map_completion_values(available);
             }