feat: add --timeout flag to scan subcommand

[mendsec]

Context

Users running catnet in CI pipelines need a hard wall-clock timeout independent of per-host timeouts. Today the only way to enforce a total time budget is to wrap the binary with \ imeout(1). A native --timeout flag would pass a context.WithTimeout to engine.StartScan and produce exit code 2 with a clear message on expiry.

Acceptance criteria

• \catnet scan 192.168.1.0/24 --timeout 30s\ aborts after 30s
• Exit code is 2 (ExitCodeRuntimeError) on timeout
• JSON output (if --format json) contains partial results collected before timeout
• --timeout flag documented in README Usage section
• Integration test covers timeout expiry path