CLI: Add format command for code formatting

[pdaxt]

Summary

Add nova fmt command to format Nova source code.

Motivation

Consistent code formatting is essential for:

• Readable codebases
• Reduced bikeshedding in code reviews
• AI-generated code that matches project style

Proposed Implementation

# Format a file
$ nova fmt src/main.nova

# Format and check (CI mode)
$ nova fmt --check src/

# Format with diff output
$ nova fmt --diff src/main.nova

Formatting Rules

• 4-space indentation
• 100 character line limit
• Trailing newline
• No trailing whitespace
• Consistent brace style

Acceptance Criteria

• nova fmt <file> formats in place
• nova fmt --check returns non-zero on unformatted code
• nova fmt --diff shows changes without applying
• Handles all current syntax elements