fix(windows): refresh terminal environment from registry on launch

[weiningwei]

Problem

When SourceGit is running and the user installs new CLI tools (Node.js, Python, Rust, etc.) or modifies system/user environment variables, clicking "Open in Terminal" opens a terminal with the stale environment snapshot from when SourceGit was started. The new PATH entries are missing, so commands from newly installed tools are not found.

This affects all Windows terminal types: cmd, pwsh, git-bash, and Windows Terminal.

Root Cause

Process.Start with default UseShellExecute = false uses CreateProcess, which inherits the parent process's Environment Block — the snapshot from SourceGit's startup time. UseShellExecute = true also does not solve this, as ShellExecuteEx with NULL lpEnvironment also inherits the parent environment.

Solution

Add RefreshEnvFromRegistry method that re-reads the latest environment from the Windows registry each time "Open in Terminal" is clicked, and sets a custom ProcessStartInfo.Environment for the child process.

Algorithm

1. Read Machine env from registry (HKLM\...\Environment)
2. Read User env from registry (HKCU\Environment) — overrides Machine, PATH is appended not replaced
3. Preserve dynamic process-only variables (TEMP, USERNAME, COMPUTERNAME, etc.) that are not in registry