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>

examples/file_io.lux

@@ -0,0 +1,50 @@
+// File I/O example - demonstrates the File effect
+//
+// This script reads a file, counts lines/words, and writes a report
+
+fn countLines(content: String): Int = {
+    let lines = String.split(content, "\n")
+    List.length(lines)
+}
+
+fn countWords(content: String): Int = {
+    let words = String.split(content, " ")
+    List.length(List.filter(words, fn(w: String): Bool => String.length(w) > 0))
+}
+
+fn analyzeFile(path: String): Unit with {File, Console} = {
+    Console.print("Analyzing file: " + path)
+
+    if File.exists(path) then {
+        let content = File.read(path)
+        let lines = countLines(content)
+        let words = countWords(content)
+        let chars = String.length(content)
+
+        Console.print("  Lines: " + toString(lines))
+        Console.print("  Words: " + toString(words))
+        Console.print("  Chars: " + toString(chars))
+    } else {
+        Console.print("  Error: File not found!")
+    }
+}
+
+fn main(): Unit with {File, Console} = {
+    Console.print("=== Lux File Analyzer ===")
+    Console.print("")
+
+    // Analyze this file itself
+    analyzeFile("examples/file_io.lux")
+    Console.print("")
+
+    // Analyze hello.lux
+    analyzeFile("examples/hello.lux")
+    Console.print("")
+
+    // Write a report
+    let report = "File analysis complete.\nAnalyzed 2 files."
+    File.write("/tmp/lux_report.txt", report)
+    Console.print("Report written to /tmp/lux_report.txt")
+}
+
+let result = run main() with {}