Add a --silent flag to suppress all terminal output

Currently, running `npx @dhanushnehru/lockcheck` prints either a terminal table or a JSON blob. For developers who want to run this in strict CI environments where they only care about the exit code (0 for pass, 1 for fail), it would be great to have a `--silent` flag.

**Acceptance Criteria:**
- Add `--silent` parsing to `bin/lockcheck.js`.
- If `--silent` is passed, do not call `printReport()` or `printJsonReport()`.
- The CLI should still `process.exit(results.exitCode)` properly.
- Document the new flag in the `--help` output and `README.md`.

Start in bin/lockcheck.js by tracing argument parsing, report output, and process.exit(results.exitCode). Check the existing --help output, then update README.md to document the flag. Done means --silent suppresses both report formats while preserving exit codes and help documentation.

Written by the indexing model from the issue text.