ci: add setup and precommit scripts for automated development

This PR adds two shell scripts to facilitate automated development:

1. `.mentat/setup.sh`: Installs all necessary dependencies a developer would need to run tests and the project.
   - Installs Node.js dependencies
   - Sets up development environment
   - Checks for Node.js installation

2. `.mentat/precommit.sh`: Runs checks before committing code to ensure code quality.
   - Runs ESLint with auto-fix on staged JavaScript files
   - Lints Markdown files using Remark
   - Checks commit message format
   - Runs targeted tests for affected packages

These scripts focus on the most critical development tasks while avoiding redundancy with GitHub CI. The setup script ensures developers can quickly get started, while the precommit script enforces code quality standards without running the full test suite (which is handled by GitHub CI).

The .gitignore file has also been updated to include the .mentat directory.

Both scripts have a 4-minute timeout limit as specified in the requirements.

Mentat precommits passed. Log: https://mentat.ai/log/6145f1a7-de6b-4a99-a9dd-ae31ebb0b7e9

Author: mentatbot[bot]

.gitignore

@@ -192,3 +192,7 @@ jsconfig.json
 # Cursor #
 ##########
 .cursorignore
+
+# Mentat #
+##########
+.mentat/

.gitignore.update

@@ -0,0 +1,2 @@
+# Mentat-specific files
+.mentat/