v0.7.4.grt: fixes for os-gated warnings, remote claude login, and premature session exit

[grtakaha]

Problem 1: Claude was stalling on remote login.

• WinSshExecChannel kept quitting prematurely and trying to reconnect. Claude and I were able to trace this back to two locations:
  * A bash command in ssh.rs with "exit" inside "{ }", which was prematurely exiting the parent shell - not just a subshell. This was fixed by using "( )" instead.
  * check_remote_claude_auth() in claude.rs might have been killing the channel by trying to source the user's bash profile during authentication. Anything that calls "exit" here kills the session. Unsure if this was a primary problem, as the change to the above bash command was the one that ultimately worked, but leaving it as-is for now (see lines 1210-1224). This was fixed by removing the source profile command and just exporting PATH. From the comment above the source profile command, it seems like this might cause problems if Claude is in an unusual location - keep an eye on it.

Problem 2: There were a handful of Rust compiler warnings related to unused and/or OS-specific code.

• This was fixed by gating or allowing "dead" code (which was mostly only dead because I was building for Windows). Note that there is a chance I overreached. Keep an eye out for missing dependencies/functions when building.