feat: add File and Process effects for real I/O

Adds two essential effects that enable Lux to interact with the system:

File effect:
- read(path) - Read file contents as string
- write(path, content) - Write string to file
- append(path, content) - Append to file
- exists(path) - Check if file/directory exists
- delete(path) - Delete a file
- readDir(path) - List directory contents
- isDir(path) - Check if path is directory
- mkdir(path) - Create directory (including parents)

Process effect:
- exec(cmd) - Run shell command, return stdout
- execStatus(cmd) - Run command, return exit code
- env(name) - Get environment variable (returns Option)
- args() - Get command line arguments
- exit(code) - Exit program with code
- cwd() - Get current working directory
- setCwd(path) - Change working directory

Also fixes formatter bug with empty handler blocks in `run ... with {}`.

These effects make Lux capable of writing real CLI tools and scripts.

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

src/typechecker.rs

@@ -935,7 +935,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference
@@ -1440,7 +1440,7 @@ impl TypeChecker {
 
         // Built-in effects are always available in run blocks (they have runtime implementations)
         let builtin_effects: EffectSet =
-            EffectSet::from_iter(["Console", "Fail", "State", "Reader", "Random", "Time"].iter().map(|s| s.to_string()));
+            EffectSet::from_iter(["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process"].iter().map(|s| s.to_string()));
 
         // Extend current effects with handled ones and built-in effects
         let combined = self.current_effects.union(&handled_effects).union(&builtin_effects);