Use the remote user's login shell

[nickadminroot]

Summary

• replace the hardcoded bash --rcfile ... command with the remote account's executable $SHELL, falling back to the passwd login shell and /bin/sh
• start Bash, Zsh, and Fish with their native login/startup semantics while installing shell-specific VS Code environment refresh hooks
• proxy Zsh's .zshenv, .zprofile, .zshrc, and .zlogin in order without duplicate sourcing, and restore the original ZDOTDIR state
• preserve tmux's standard SSH/display update-environment variables alongside the VS Code variables
• reload changed tmux configuration with source-file instead of destroying persistent sessions with kill-server
• add automated shell/tmux integration tests and a local ESLint/VSCE toolchain

Existing panes intentionally keep the shell process they were created with; newly created panes use the selected login shell.

For Bash, profiles that customize PROMPT_COMMAND should preserve its inherited value so Persisterm can continue refreshing VS Code IPC variables after reconnects.

Testing

• npm run lint
• npm test — 8/8 passing, including real tmux panes for Bash, Zsh, and Fish
• npm audit --audit-level=high — 0 vulnerabilities
• npm run package -- --out persisterm-user-shell.vsix
• manual VSIX verification with Zsh over VS Code Remote SSH

Closes #1