Add simple text config file support

[ammario]

Summary

• allow specifying filtering rules via --config pointing to a plain text file
• document new rules.txt format with per-line rules and examples
• test reading rules from config files

Testing

• cargo clippy --all-targets -- -D warnings
• cargo test (fails: Failed to execute ip netns add: No such file or directory)

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https://chatgpt.com/codex/tasks/task_e_68c1ae34e42c8329bd3442577d1d8fd8