fix(cli): detect Bun on Windows in launcher (which → where)

[Trypd]

Problem

On Windows, claudish fails to launch even when Bun is installed and on PATH:

'which' is not recognized as an internal or external command,
operable program or batch file.
claudish requires the Bun runtime but it was not found.

bin/claudish.cjs findBun() is POSIX-only:

1. execSync("which bun") — which doesn't exist in cmd/PowerShell, so the lookup throws.
2. The fallback candidates are all Unix paths ($HOME/.bun/bin/bun, /usr/local/bin/bun, /opt/homebrew/bin/bun) — none match the Windows install location %USERPROFILE%\.bun\bin\bun.exe.
3. process.env.HOME is typically unset on Windows (it's USERPROFILE), so even the first candidate resolves to undefined/.bun/bin/bun.

Net effect: a correctly installed Bun is reported as missing on every Windows machine.

Fix

• Use where bun on win32, which bun elsewhere; take the first returned line.
• Add Windows fallback candidates %USERPROFILE%\.bun\bin\bun.exe (and extensionless), using HOME || USERPROFILE.
• Make the "install Bun" hint platform-aware — Windows users get the PowerShell one-liner (irm bun.sh/install.ps1 | iex) instead of curl … | bash.

No behavior change on macOS/Linux (same which path and same Unix candidates).

Testing

• Windows 11 + Bun 1.3.13 on PATH: claudish --version → claudish version 7.1.2 (was failing before).
• node --check bin/claudish.cjs passes.