Make /skills install reusable skill packs

The Rust commands layer could list skills, but it had no concrete install path.
This change adds /skills install <path> and matching direct CLI parsing so a
skill directory or markdown file can be copied into the user skill registry
with a normalized invocation name and a structured install report.

Constraint: Keep the enhancement inside the existing Rust commands surface without adding dependencies
Rejected: Full project-scoped registry management | larger parity surface than needed for one landed path
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If project-scoped skill installation is added later, keep the install target explicit so command discovery and tool resolution stay aligned
Tested: cargo test -p commands
Tested: cargo clippy -p commands --tests -- -D warnings
Tested: cargo test -p rusty-claude-cli parses_direct_agents_and_skills_slash_commands
Tested: cargo test -p rusty-claude-cli parses_login_and_logout_subcommands
Tested: cargo clippy -p rusty-claude-cli --tests -- -D warnings
Not-tested: End-to-end interactive REPL invocation of /skills install against a real user skill registry

rust/crates/rusty-claude-cli/src/main.rs

@@ -1,4 +1,11 @@
-#![allow(dead_code, unused_imports, unused_variables, clippy::unneeded_struct_pattern, clippy::unnecessary_wraps, clippy::unused_self)]
+#![allow(
+    dead_code,
+    unused_imports,
+    unused_variables,
+    clippy::unneeded_struct_pattern,
+    clippy::unnecessary_wraps,
+    clippy::unused_self
+)]
 mod init;
 mod input;
 mod render;
@@ -5181,6 +5188,17 @@ mod tests {
                 args: Some("help".to_string())
             }
         );
+        assert_eq!(
+            parse_args(&[
+                "/skills".to_string(),
+                "install".to_string(),
+                "./fixtures/help-skill".to_string(),
+            ])
+            .expect("/skills install should parse"),
+            CliAction::Skills {
+                args: Some("install ./fixtures/help-skill".to_string())
+            }
+        );
         let error = parse_args(&["/status".to_string()])
             .expect_err("/status should remain REPL-only when invoked directly");
         assert!(error.contains("interactive-only"));