Add thinktank init command for guided first-run onboarding

[that-github-user]

Summary

New users have to figure out thinktank from the README. A guided thinktank init command would:

1. Check if Claude Code CLI is installed
2. Detect the project's test command (package.json scripts, Makefile, etc.)
3. Create .thinktank/config.json with sensible defaults
4. Run a small demo task to verify everything works
5. Show a "You're all set!" message with next steps

Why

First impression matters for GitHub stars. A smooth onboarding experience = more adoption.

[that-github-user]

Fixed in #151. thinktank init checks Claude Code CLI, git repo, detects test command, sets defaults.