feat(capture): add --duration parameter with auto-save to temp pcap

[luongnv89]

Summary

Addresses issue #100: Add --duration parameter to the capture command with auto-save to temp file.

Changes

New -d/--duration <seconds> option for capture command

• Runs a timed capture loop that stops automatically after the specified duration
• Saves captured packets to a temp pcap file at /tmp/mmtreader-<pid>.pcap
• Prints the temp file path to stdout so agents can reference it
• Graceful shutdown on SIGINT/SIGTERM (existing Ctrl+C behavior preserved)
• No --duration flag = existing infinite capture behavior unchanged

Bug fix

• Fixed missing colon after F in getopt string (regression from adding d:)

Acceptance Criteria

• --duration <seconds> parameter accepted by the capture command and documented in --help
• Capture loop stops automatically after the specified duration, with graceful shutdown
• Captured packets are saved to a temp pcap file by default
• The temp file path is printed to stdout
• Existing behavior (Ctrl+C to stop) is preserved when no --duration is given

Testing

• All 350 unit + integration tests pass
• Build is warning-free (-Wall -Wextra)
• CLI integration tests pass