refactor: split Terminal into PtyStream + ChildHandle

Decompose the monolithic Terminal struct into focused components:
- PtyStream: combined reader/writer (impl Read + Write) with vt100
  parser, screen_contents(), resize(), send_ctrl_c(), write_line()
- ChildHandle: cloneable child process handle with wait() and kill()
- Terminal: thin wrapper with pub pty_stream and child_handle fields

Drop custom read_until/read_to_end methods in favor of std::io traits.
Remove 7 read_until-specific tests; adapt remaining 9 tests to use
BufReader<&mut PtyStream>::read_until for synchronization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: branchseer