Add installer and startup update prompt

[philipptpunkt]

Summary

• Adds a one-command Linux/macOS installer that downloads the matching release asset into ~/.github-butler and wires PATH.
• Checks GitHub releases before the TUI starts and prompts users to self-update when a newer version is available.
• Restarts github-butler automatically with the same CLI args after a successful update.
• Updates the README install flow to match the new installer and documents the update prompt.

Test plan

• bash -n scripts/install.sh
• gofmt -w cmd/update_prompt.go internal/update/update.go internal/update/restart_unix.go internal/update/restart_windows.go internal/update/update_test.go
• go test ./...
• GOOS=windows GOARCH=amd64 go build ./...
• go run . --version