Learning panel follow-up

[minestarks]

DON'T REVIEW will split into separate PRs.

Follow-up to #3228. Addresses review feedback items.

Hint format migration

• Exercise hint blocks in all 49 kata index.md files converted from <details><summary>Need a hint</summary>…</details> to lightweight :::hint / ::: fenced markers.
• generate_katas_content.js updated to extract hints using the new multiline regex. Legacy <details> pattern kept as fallback.

Agent model fallback

• qdk-learning.agent.md now specifies model: ["Claude Haiku 4.5 (copilot)", "auto"] so the agent works even when the preferred model is unavailable.

Corrupt progress file handling

• Previously, a corrupt or malformed qdk-learning.json was silently treated as "missing" and reset to defaults (losing progress without warning).
• Now distinguishes file-not-found (uses defaults) from file-exists-but-corrupt (shows an error message and blocks initialization). Error state is surfaced via progressFileError so the LM tool can advise the user.

Tree view click-to-navigate

• Clicking any tree item (unit, activity, or "Up next") now directly opens the lesson panel via item.command. Previously required clicking a separate inline action icon.
• The "Go to Activity" inline button is removed from the tree view.

Chat button behavior

• "Ask in Chat" button on tree items now navigates to the activity first, then opens chat with a friendly prompt (Let's work on the "…" exercise.). Previously sent a raw /qdk-learning #goto … command without navigating.

Progress dots

• Removed per-segment title tooltips from the progress bar dots — they were misleading since individual dots are not clickable.

Exercise reset from panel

• Added a "Reset" action button to the exercise action bar in the webview panel.
• Triggers a native VS Code modal confirmation dialog before resetting.

Azure Quantum panel location

• quantum-workspaces tree view moved from Explorer sidebar into the "Microsoft Quantum" activity bar container.
• A placeholder view remains in Explorer directing users to the new location.

Folder consolidation

• agents/, prompts/, skills/, resources/chat-instructions/ consolidated into a single ai directory. package.json paths updated.