fix(cli): validate and reject misplaced flags placed after '--'

[HarshK97]

Problem

1. When flags are placed after -- (e.g. diffm main path -- -f sbs), Cobra stops parsing options and dumps the flag into positional arguments.
2. The CLI then tries to resolve the flag as a Git revision or file path, resulting in confusing downstream errors instead of explaining why the flag was ignored.

What Changed

• Added validateNoMisplacedFlags to check positional arguments for arguments starting with -.
• Allowed valid positional arguments through, specifically stdin ("-") and actual files on disk or in the Git index that start with -.
• Ran the validation at the very start of rootCmd.Run so misplaced flags fail fast across all modes, including --parse-tree and --cst.
• Added table-driven unit tests in cmd/diff_test.go covering valid paths, stdin, misplaced short/long flags, and disk files starting with a dash.