refactor: Restructure multi-language workflow plugins to match Python pattern

Each plugin now follows the standardized structure:
- category/plugin_name/plugin_name.{ext} - Implementation
- category/plugin_name/package.json - Plugin metadata
- category/package.json - Category manifest with "plugins" array

Languages restructured:
- Go: 31 plugins across 7 categories (math, string, logic, list, dict, var, convert)
- Rust: 55 plugins as individual crates in Cargo workspace
- C++: 34 header-only plugins with complete coverage
- Mojo: 11 plugins across 3 categories (math, string, list)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

workflow/plugins/rust/string/string_substring/src/lib.rs

@@ -0,0 +1,57 @@
+//! Workflow plugin: substring.
+
+use serde_json::Value;
+use std::collections::HashMap;
+
+/// Extract a substring from a string.
+pub fn run(_runtime: &mut HashMap<String, Value>, inputs: &HashMap<String, Value>) -> Result<HashMap<String, Value>, String> {
+    let string: String = inputs
+        .get("string")
+        .and_then(|v| serde_json::from_value(v.clone()).ok())
+        .unwrap_or_default();
+    let start: i64 = inputs
+        .get("start")
+        .and_then(|v| serde_json::from_value(v.clone()).ok())
+        .unwrap_or(0);
+    let end: Option<i64> = inputs
+        .get("end")
+        .and_then(|v| serde_json::from_value(v.clone()).ok());
+
+    let chars: Vec<char> = string.chars().collect();
+    let len = chars.len() as i64;
+
+    // Handle negative indices
+    let start_idx = if start < 0 { (len + start).max(0) } else { start.min(len) } as usize;
+    let end_idx = match end {
+        Some(e) if e < 0 => (len + e).max(0) as usize,
+        Some(e) => e.min(len) as usize,
+        None => len as usize,
+    };
+
+    let result: String = if start_idx < end_idx {
+        chars[start_idx..end_idx].iter().collect()
+    } else {
+        String::new()
+    };
+
+    let mut output = HashMap::new();
+    output.insert("result".to_string(), serde_json::json!(result));
+    Ok(output)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_substring() {
+        let mut runtime = HashMap::new();
+        let mut inputs = HashMap::new();
+        inputs.insert("string".to_string(), serde_json::json!("hello world"));
+        inputs.insert("start".to_string(), serde_json::json!(0));
+        inputs.insert("end".to_string(), serde_json::json!(5));
+
+        let result = run(&mut runtime, &inputs).unwrap();
+        assert_eq!(result.get("result"), Some(&serde_json::json!("hello")));
+    }
+}