feat(install): rename old manual binaries to .bak after Homebrew install (#8)

When migrating from manual install to Homebrew, rename existing
~/.shelltime/bin/shelltime and shelltime-daemon with .bak suffix so
the shell resolves to the Homebrew-managed version instead.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: AnnatarHe

install.bash

@@ -24,6 +24,15 @@ if [[ "$OS" == "Darwin" ]] && command_exists brew; then
     if brew install shelltime/tap/shelltime; then
         BREW_INSTALLED=true
         echo "Successfully installed shelltime via Homebrew."
+        # Rename old manual-install binaries so the system uses the Homebrew version
+        if [ -f "$HOME/.shelltime/bin/shelltime" ]; then
+            mv "$HOME/.shelltime/bin/shelltime" "$HOME/.shelltime/bin/shelltime.bak"
+            echo "Renamed ~/.shelltime/bin/shelltime to shelltime.bak (now using Homebrew version)"
+        fi
+        if [ -f "$HOME/.shelltime/bin/shelltime-daemon" ]; then
+            mv "$HOME/.shelltime/bin/shelltime-daemon" "$HOME/.shelltime/bin/shelltime-daemon.bak"
+            echo "Renamed ~/.shelltime/bin/shelltime-daemon to shelltime-daemon.bak (now using Homebrew version)"
+        fi
     else
         echo "Homebrew installation failed. Falling back to manual installation..."
     fi