Configure Git Bash shell profiles during Windows install

Summary

Fixes #517. Git Bash (MSYS2/MinGW) prepends /mingw64/bin to PATH during its own shell initialization, which shadows the git-ai shim even when the Windows PATH has been correctly set by Set-PathPrependBeforeGit. This adds a new section to install.ps1 that writes export PATH="$HOME/.git-ai/bin:$PATH" into the user's ~/.bashrc or ~/.bash_profile, mirroring what install.sh already does for Unix shells.

The new block:

1. Detects Git for Windows by checking common install paths (Program Files, Program Files (x86), LocalAppData\Programs), guarding each path against null env vars
2. Finds existing .bashrc or .bash_profile (or creates .bashrc if neither exists)
3. Checks for an existing .git-ai/bin marker to avoid duplicate entries
4. Appends the PATH export using BOM-less UTF-8 encoding (via [System.IO.File]::AppendAllText with UTF8Encoding($false))
5. Wraps everything in try/catch so a failure here never blocks the rest of the install

Updates since last revision

• Fixed UTF-8 BOM issue: replaced Add-Content -Encoding UTF8 with [System.IO.File]::AppendAllText() using UTF8Encoding($false) to avoid writing a BOM that would cause command not found errors in bash on PowerShell 5.1.
• Fixed null env var issue: Join-Path calls for Git Bash detection paths are now guarded with if checks so a null ProgramFiles(x86) (e.g. on 32-bit Windows) doesn't abort the entire Git Bash config block under Set-StrictMode.

Review & Testing Checklist for Human

• Test on a real Windows machine with Git Bash installed — this change is entirely in PowerShell and cannot be verified by the Rust test suite or Linux CI. Run the installer, then open Git Bash and confirm which git resolves to ~/.git-ai/bin/git (not /mingw64/bin/git)
• Verify no UTF-8 BOM in created .bashrc — the code now uses UTF8Encoding($false) + AppendAllText, but confirm the written file has no BOM bytes (EF BB BF) at the start, especially when .bashrc is newly created (no pre-existing file)
• Test idempotency — run the installer twice and confirm the PATH export is not duplicated in .bashrc
• Test with no existing .bashrc — confirm the installer creates .bashrc and the export line is correct. Consider whether creating .bashrc when a user has never used Git Bash is desirable, or if this should only append to existing files

Notes

• Git Bash detection only covers standard Git for Windows install paths. Custom locations (Scoop, Chocolatey portable, etc.) won't be detected, but the code simply skips configuration in that case — no harm done.
• No admin permissions required — writes only to user-scoped files (%USERPROFILE%\.bashrc / .bash_profile).
• Link to Devin run
• Requested by @svarlamov