fix(installer): reuse existing whispercpp builds (#421)

docs/INSTALL.md

@@ -103,6 +103,8 @@ Options:
   --test           Run tests after installation
   --venv-dir=PATH  Specify custom virtual environment directory
   --skip-models    Skip downloading speech models during installation
+  --rebuild-whispercpp     Rebuild/reinstall pywhispercpp even if already installed
+  --no-rebuild-whispercpp  Reuse existing pywhispercpp when present (auto-mode default)
   --tag=TAG        Install specific release tag
   --help           Show this help message
 
@@ -360,6 +362,17 @@ vulkaninfo --summary | grep -i "deviceName"
 # [INFO] whisper.cpp using Vulkan GPU backend: AMD Radeon RX 6800
 ```
 
+### Reusing Existing whisper.cpp Builds
+
+When updating an existing install, the installer checks for a working `pywhispercpp`
+installation before rebuilding it. Interactive installs ask whether to rebuild, with
+the default set to no. Automatic installs reuse the existing build by default.
+
+```bash
+./install.sh --auto                         # Reuse pywhispercpp if already installed
+./install.sh --auto --rebuild-whispercpp    # Force a rebuild/reinstall
+```
+
 ### Switching Engines
 
 If you want to try a different engine after installation: