second exercise!

- create exercise string manipulator (shout out claude)
- finish hacky solution (it almost works)

exercises/string-manipulator/string-manipulator-exercise.md

@@ -0,0 +1,30 @@
+# Exercise: String Manipulator Tool
+
+**Concepts practiced:** String vs &str, ownership/borrowing, Vec operations, pattern matching
+**Objective:** Build a CLI tool that performs various string operations
+
+## Requirements:
+- Create a `StringTool` struct that holds a collection of strings
+- Implement methods:
+  - `new()` - creates empty tool
+  - `add_string(&mut self, s: String)` - adds string to collection
+  - `longest(&self) -> Option<&str>` - returns longest string (or None if empty)
+  - `contains_word(&self, word: &str) -> Vec<&str>` - returns strings containing the word
+  - `to_uppercase_all(&mut self)` - converts all strings to uppercase in-place
+
+## Success criteria:
+- Code compiles without warnings
+- Test with: adding "hello", "world", "rust programming"
+- `longest()` should return "rust programming"
+- `contains_word("o")` should return vec!["hello", "world", "rust programming"]
+- After `to_uppercase_all()`, all strings should be uppercase
+
+## Stretch goal:
+Add a method `word_count(&self) -> HashMap<String, usize>` that counts total occurrences of each word across all strings
+
+## Getting Started:
+1. `cd exercises && cargo new string-manipulator`
+2. Focus on ownership: when do you need `String` vs `&str`?
+3. Think about borrowing: methods that read vs modify
+
+Expected time: 45-60 minutes