Agent panel: resolve file:// image URLs and fall back to a link for unresolved inline images

[nad182]

Summary

Make inline images in agent responses more forgiving: resolve file:// URLs in ![alt](...), and fall back to a clickable link instead of bare alt text when an image can't be resolved.

Current behavior

Zed 1.21.0 (stable), macOS. The agent panel already renders ![alt](/absolute/path.png), ![alt](relative/to/worktree.png) and ![alt](https://...) inline (resolve_agent_image in crates/agent_ui/src/agent_ui.rs), which is great. Two gaps make it easy to miss in practice:

1. ![alt](file:///Users/me/repo/.qa-screenshots/dev_auth.png) is not resolved. The panel shows only the text dev_auth screen, with nothing to click. file:// is the form agents naturally produce, because the same panel renders [name](file:///...) links as clickable, so instructions like "use file:// links for files" spill over into image syntax. I hit this myself and first thought images were not supported at all.
2. When the image can't be resolved (bad path, unsupported scheme), the fallback is plain alt text. A clickable link to dest_url would at least let me open the file in a tab.

Attaching images to the agent works fine, this is only about images in the agent's output.

Expected behavior

• resolve_agent_image accepts file:// URLs (strip the scheme, percent-decode, then treat as an absolute path).
• Unresolved images render as a clickable link (alt text as label, dest_url as target) instead of bare text.
• Optional: a bare inline image (not wrapped in a link) opens in the image viewer on click, so agents don't need the [![alt](/abs.png)](file:///abs.png) double-wrap to get click-to-open.

Why

Agent workflows often end with a screenshot: browser QA through the Playwright MCP, visual regression checks, before / after captures for PR descriptions. The rendering is already there; it just needs to accept the URL form agents produce and degrade to a link instead of silently dropping the image.

I'm happy to open a PR for this if it sounds right.

Related but different: #58491 (image mentions in restored threads), #50658 (pasting images into the panel), #64352 (drag-and-drop attachments). Those are about input, not rendering agent output.