refactor: replace cobra subcommands with pflag/viper flags

[yuuki]

Summary

This PR refactors the CLI from cobra subcommands to simple pflag/viper flags, replacing tcpulse connect and tcpulse serve with tcpulse -c and tcpulse -s respectively.

Key Changes

• Simplified CLI: Use -c/--client and -s/--server flags instead of subcommands
• Removed Dependencies: Eliminated cobra dependency, using only pflag and viper
• Consolidated Architecture: Moved all CLI logic from separate files into main.go
• Maintained Functionality: All existing features and options preserved
• Updated Tests: Adapted test suite to work with new flag-based interface

Benefits

• Simpler command-line interface that's more intuitive
• Reduced dependencies and smaller binary size
• Cleaner codebase with consolidated CLI logic
• Better alignment with POSIX-style flag conventions

Migration Guide

Before:

tcpulse serve -l 0.0.0.0:8080
tcpulse connect -c 10 -d 30s localhost:8080

After:

tcpulse -s 0.0.0.0:8080
tcpulse -c --connections 10 --duration 30s localhost:8080

Test plan

• Build succeeds without errors
• Server mode works with -s flag
• Client mode works with -c flag
• All existing flags and options functional
• Version flag works: tcpulse --version
• Help output shows correct usage: tcpulse --help
• Error handling for missing mode selection
• Integration test: server and client communication successful

🤖 Generated with Claude Code

[openhands-ai]

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@OpenHands please fix the failing actions on PR #28

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