chore: using mise to manage node versions and tasks (#44)

* add mise for Node.js version management

Replace ad-hoc Node version pinning with mise (mise.toml + mise.lock).
This ensures all contributors and CI use the same Node LTS version.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* expand mise with env loading, PATH management, and task runner

- [env] _.file loads .env files for local API keys (replaces dotenv)
- [env] _.path adds node_modules/.bin to PATH for direct tool access
- [tasks] mirrors npm scripts with sources/outputs for file-based caching
- eval tasks for running LLM evaluations via mise run
- update AGENTS.md and CONTRIBUTING.md to use mise run commands

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -2,6 +2,7 @@ node_modules/
 dist/
 *.log
 .DS_Store
+.env
 
 # IDE/Editor directories
 .vscode/