feat: Add tmux integration with multi-backend support

[Braddon]

Summary

This PR implements comprehensive tmux and GNU Screen support through a function-based abstraction layer, allowing hip-screen to seamlessly work with both terminal multiplexers.

Changes

Core Implementation

• Backend detection: Auto-detects available multiplexer with tmux preference
• 9 abstraction functions: Encapsulate all backend-specific operations
• Environment variables: $HS_BACKEND to override, $HS_DEBUG for diagnostics
• Session name validation: Prevents invalid tmux names with helpful error messages

Backend-Specific Optimizations

• tmux: Uses native format variables (#{session_name}, #{session_attached}, #{session_activity})
• screen: Uses socket file stats and ps parsing (maintains compatibility)

UI Updates

• Header now shows active backend: hip-screen (tmux) or hip-screen (screen)
• Session name validation prevents tmux errors upfront
• Connection counts and last activity work with both backends

Documentation

• Updated CLAUDE.md with complete backend architecture details
• Rewrote README.md with backend selection guide, requirements, and troubleshooting
• Added testing procedures for both backends

Breaking Changes

• Header format changed from "There are N screen(s)" to "hip-screen (BACKEND) - N session(s)"
• When using tmux, session names cannot contain periods (.), colons (:), or spaces (validation added)

Testing

Automated

• ✅ Bash syntax check passes
• ✅ Both backends detected correctly
• ✅ Environment variable overrides work

Manual

• ✅ tmux backend lists sessions correctly
• ✅ screen backend lists sessions (including ones with spaces)
• ✅ Current session highlighting works in both backends
• ✅ Connection counts accurate
• ✅ Last activity times display correctly
• ✅ Backend indicator shows in header

Backward Compatibility

• Existing screen users experience no changes (unless tmux is installed, then need export HS_BACKEND=screen)
• All existing screen functionality preserved
• Session names with spaces still work in screen backend

Migration Guide

For screen users who want to keep using screen:

export HS_BACKEND=screen

For users wanting to switch to tmux:

• Install tmux: sudo apt install tmux (or brew/yum)
• Run hs - will auto-detect tmux
• Use hyphens/underscores in session names instead of spaces/periods/colons