Refactor shell I/O handling with unified stdin reader

[mariotaku]

Summary

This PR refactors the shell I/O handling in both PTY and dumb modes to use a unified, cleaner approach for reading stdin and managing the event loop. The changes extract common I/O patterns into a new io module and simplify the main shell loops.

Key Changes

• New io module: Extracted spawn_stdin_reader() function that reads stdin on a background thread and forwards bytes through a channel, replacing the previous per-mode event handling threads

• Simplified PTY mode (pty.rs):

  • Removed complex EventThread struct and crossterm event handling
  • Replaced with simple stdin reader + ticker-based polling
  • Introduced RawMode RAII guard to ensure terminal is restored even on early return
  • Consolidated remote output reading into drain() helper function
  • Increased buffer size from 1KB to 8KB for better throughput

• Simplified dumb mode (dumb.rs):

  • Removed EventThread struct in favor of shared spawn_stdin_reader()
  • Added crlf_to_lf() function to normalize line endings (CR/CRLF → LF) since dumb mode lacks remote line discipline
  • Includes comprehensive tests for line ending conversion, including edge cases with splits across chunks
  • Consolidated remote output reading into drain() helper function
  • Increased buffer size from 1KB to 8KB

• Unified polling: Both modes now use crossbeam_channel::tick() for periodic polling with a 10ms interval, replacing ad-hoc sleep calls

• Error handling: Extracted io_error() helper for consistent error conversion from libssh errors

• Terminal handling: Improved TERM environment variable handling in main.rs

Implementation Details

• The stdin reader thread runs indefinitely, reading into an 8KB buffer and sending chunks through an unbounded channel
• Both shell modes now use select! to multiplex between stdin input and a periodic ticker
• When local stdin reaches EOF, send_eof() is called on the remote channel to signal completion
• The dumb mode's crlf_to_lf() function maintains state across chunk boundaries to handle CRLF pairs split across reads
• All remote output is drained non-blockingly before checking for input, ensuring responsive output display

https://claude.ai/code/session_017vAGzSSYNcgdQzJRLWfX47