fix: tighten Start dialogs, hands-on cwd, and hint prompts

[jiaxinnns]

Overview

Start's intro did not name the activity, the missing-CLI dialog was a checklist, hands-on Start stopped in hp-* instead of the sandbox repo, and AI Hints offered different prompts per kind. Start now names the activity, points missing CLI at the lesson, cds into the hands-on repo, and both kinds share two hint openers.

Solution

• Intro title is Starting <name>, using the same display names as the left nav
• Missing-CLI step is only that title and a "here" link to T1L2; the next Start re-checks
• Hands-on Start cds into the single subdirectory when there is one (things, a clone), same role as an exercise repo_name
• Exercises and hands-on share two AI hint suggestions

Test cases

• Start an exercise with the intro still enabled: title is Starting Under Control (or that exercise's name)
• Start a detour: title uses the detour name, e.g. Starting Undoing a Repo Initialization
• Start a hands-on: title is Starting Init Repo (or that practical's name)
• Uninstall the CLI, then Start: dialog title is Git-Mastery CLI was not found., subtitle has a here link, no Check again or Setup instructions buttons
• Click here: modal closes and the lesson opens at the T1L2 setup section
• After installing the CLI, Start again continues the flow
• Start Hands-on for hp-view-commits: terminal cds into hp-view-commits/things, not the hp-* folder
• Open AI Hints on an exercise and a hands-on: both show only "I'm stuck. What should I do next?" and "Explain the underlying concepts to me"