Replace .NET WinForms handler with the Rust build in CI; drop SshHandler

Remove the .NET/WinForms project (SshHandler/, SshHandler.sln) now that
the Rust rewrite (SshHandlerRs/) covers the same behavior with a
~250KB binary and no CLR startup cost, and its Default-Apps-style
registration is confirmed working.

Rewire the release workflow to build ssh-handler.exe natively on
windows-latest with the MSVC toolchain instead of dotnet publish, and
package it alongside the .bat and .sh handlers as release assets
(ssh-handler-rs-*.exe, openssh_protocol_handler-*.bat,
ssh-handler-*.sh) when a v* tag is pushed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GYCT2RpUjFfBfp6CD4rREw

.gitignore

@@ -1,24 +1,4 @@
 # .gitignore
 
-# User-specific files
-*.rsuser
-*.suo
-*.user
-*.userosscache
-*.sln.docstates
-
-# Build results
-[Dd]ebug/
-[Dd]ebugPublic/
-[Rr]elease/
-[Rr]eleases/
-x64/
-x86/
-[Aa][Rr][Mm]/
-[Aa][Rr][Mm]64/
-bld/
-[Bb]in/
-[Oo]bj/
-
 # Rust
 /SshHandlerRs/target/