fix: use the learner's own CLI and exercises folder

[jiaxinnns]

Overview

The app was installing the CLI into the learner's save folder and putting that folder first on PATH, so a stray executable could shadow git. It now uses a CLI the learner installed themselves, and asks where they ran gitmastery setup.

Solution

• Resolve gitmastery from the normal PATH, plus Homebrew prefixes on macOS. The app no longer downloads or installs the CLI.
• Drop onboarding and the setup checklist. The first Start shows a short intro, then checks the CLI (and Git Bash on Windows), then asks for the exercises folder.
• Accept a folder that contains .gitmastery.json, or a parent with exactly one such folder. Setup Instructions in that copy opens T1L2 inside the app. An X forgets the link without deleting files.

Test cases

• A fresh launch opens the lessons, with no onboarding screen
• Start with no CLI shows the tools step; Setup instructions opens T1L2 in the app, not a browser
• The folder step says this does not set up Git-Mastery; the Setup Instructions link opens T1L2 and closes the dialog
• Picking the folder where gitmastery setup was run is accepted; a folder without .gitmastery.json is rejected
• The X unlinks the path and clears progress; the folder is still on disk
• After the folder is linked, Start downloads if needed and moves into the exercise; Verify runs
• On Windows, the tools step says to restart the app and has no Check again button