Accept $skill invocation form in Skill tool

Teach Skill path resolution to accept the common $skill invocation form in addition to bare names and /skill prefixes. Keep the behavior narrow and add regression coverage using the existing help skill fixture.

Constraint: Must not touch unrelated dirty api files in this worktree
Constraint: Keep the change limited to rust/crates/tools
Rejected: Canonicalize the returned skill field to the resolved name | would change caller-visible output semantics unnecessarily
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep invocation-prefix normalization aligned with how prompt and skill references are written elsewhere in the CLI
Tested: cargo test -p tools
Not-tested: CODEX_HOME layouts with unusual symlink arrangements

rust/crates/tools/src/lib.rs

@@ -1090,7 +1090,7 @@ fn todo_store_path() -> Result<std::path::PathBuf, String> {
 }
 
 fn resolve_skill_path(skill: &str) -> Result<std::path::PathBuf, String> {
-    let requested = skill.trim().trim_start_matches('/');
+    let requested = skill.trim().trim_start_matches('/').trim_start_matches('$');
     if requested.is_empty() {
         return Err(String::from("skill must not be empty"));
     }
@@ -1961,6 +1961,21 @@ mod tests {
             .as_str()
             .expect("prompt")
             .contains("Guide on using oh-my-codex plugin"));
+
+        let dollar_result = execute_tool(
+            "Skill",
+            &json!({
+                "skill": "$help"
+            }),
+        )
+        .expect("Skill should accept $skill invocation form");
+        let dollar_output: serde_json::Value =
+            serde_json::from_str(&dollar_result).expect("valid json");
+        assert_eq!(dollar_output["skill"], "$help");
+        assert!(dollar_output["path"]
+            .as_str()
+            .expect("path")
+            .ends_with("/help/SKILL.md"));
     }
 
     #[test]