Add --output flag to ctx diff for saving reports to file

[selwyntheo]

Currently ctx diff only prints to stdout. Add an --output <file> flag to write the diff report to a file.

Acceptance criteria:

• ctx diff <a> <b> continues to print to stdout
• ctx diff <a> <b> --output report.json writes JSON output to the specified file
• ctx diff <a> <b> --human --output report.txt writes human-readable output to file
• Add a test case

Files to look at:

• cmd/ctx/commands.go — diff command definition
• internal/diff/ — diff and output logic