Improve "AI-powered debugging" on dashboard page

[JamesNK]

The copilot content has been removed from the dashboard overview page. It now only mentions MCP server. But the screenshot is out of date, it shows the MCP server hosted inside the dashboard.

The overview page should still mention the MCP but have a fixed image. And it should have content section on using agents with the dashboard and link here: /dashboard/ai-coding-agents/

The screenshot could maybe be something like GH Copilot CLI saying that it is calling Aspire to get data and then displaying reasoning or results with that data.

[IEvangelist]

Triage → @Copilot

Scope — concrete edits to the dashboard overview page (src/frontend/src/content/docs/dashboard/overview.mdx, plus the JA mirror under ja/dashboard/overview.mdx if present):

1. Find the section that previously covered Copilot integration and now mentions only the MCP server.
2. Keep the MCP server section, but:
   • Add a new subsection titled "Using agents with the dashboard" with a one-paragraph summary and a link to /dashboard/ai-coding-agents/.
3. Replace the out-of-date screenshot. The current image shows the (removed) in-dashboard Copilot UI. The new screenshot should show an external agent (e.g. GitHub Copilot CLI) calling Aspire to fetch dashboard data and displaying the reasoning/result, not a chat panel hosted inside the dashboard UI.

Image handling

• If you can produce the screenshot yourself, drop it under src/frontend/src/assets/dashboard/ and reference it from the MDX.
• If not, leave a clearly-marked TODO(screenshot) comment in the MDX next to the old image reference and remove the out-of-date image so the page renders without a broken/misleading screenshot. Note the missing asset in the PR description so a human can supply it.

Acceptance

• Page builds cleanly (npm run build in src/frontend/).
• No remaining references to in-dashboard Copilot UI on this page.
• "Using agents with the dashboard" section exists and links to /dashboard/ai-coding-agents/.
• JA mirror updated to match (or a clearly-marked TODO added if translation needs a separate pass).
• No unrelated changes.