Fix: Remove duplicate precompile_hook key in shakapacker.yml

The spec/dummy/config/shakapacker.yml had two `precompile_hook` entries
which caused a YAML parsing error:
  YAMLException: duplicated mapping key (31:3)

Removed the first instance at line 14, keeping the one with better
documentation at line 31.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/dummy/config/shakapacker.yml

@@ -9,10 +9,6 @@ default: &default
   # SWC has issues with PropTypes handling
   javascript_transpiler: babel
 
-  # Hook to run before compilation (e.g., for ReScript builds, pack generation)
-  # See: https://github.com/shakacode/shakapacker/blob/main/docs/precompile_hook.md
-  precompile_hook: bin/shakapacker-precompile-hook
-
   cache_path: tmp/cache/shakapacker
   webpack_compile_output: false
   ensure_consistent_versioning: true