feat: snip a region into chat, and make links clickable

Two changes: screenshot input gated on the model really reporting vision,
and a fix for URLs that arrived as code.

Links
- A fence whose body is all URLs, a lone `https://...` inline span, and a
  line that is only a URL are rewritten into [url](url) markdown links.
  Vicinae attaches only the table and strikethrough GFM extensions, so
  autolink is off and a bare URL is dead text; clickability comes from
  MarkdownModel turning CMARK_NODE_LINK into an <a href>. A fenced URL was
  a dark panel with a Copy button and no way to open it.
- promoteStandaloneCodeSpans was itself promoting a lone URL span into a
  fence, so it made this worse. Link restoration now runs first.
- The default prompt says never to fence a URL.
- Anything that is not one of those three shapes is copied through, so a
  curl invocation containing an address is never rewritten.

Screenshots
- capture.ts shells out to an auto-detected tool: grimblast, grim + slurp,
  spectacle, maim, scrot, flameshot, or screencapture on macOS. There is
  no screenshot call in @vicinae/api and Clipboard moves no binary, so the
  extension owns the capture and reads the file it wrote rather than
  guessing at the clipboard. Cancel is detected by whether an image landed
  on disk, because the tools disagree on exit codes.
- In chat: "Snip a Region" in the action panel (cmd+shift+S) attaches to
  the current conversation. Standalone: "Snip to Lam" starts a new one.
- vision.ts caches the /api/show capability answer per host and model. A
  failed check is "unknown", never "no", so a dropped connection is not
  reported as a fact about the user's model. Actions still fail closed.
- Only the path is stored, base64 is read at send time, and the image goes
  on its own turn only. Capture is never a model-callable tool.

Verified: 244 tests, tsc, eslint, and vici build (both entrypoints).

src/lib/constants.ts

@@ -36,6 +36,8 @@ Write plainly:
 
 Fence every command and every snippet, and always name the language: bash for shell and terminal commands, then json, sql, yaml, or the language for code (ts, js, python, rust, go). Never leave code inline in a sentence, and never open a fence without a language. Put no $ or shell prompt inside the block so it can be copied and run as-is. Ignore the formatting of earlier answers in this conversation: if one put a command in inline code, fence it anyway.
 
+A web address is a link, never code. A markdown link, written as [label](https://example.com), is clickable in this window, while a URL inside backticks or a fenced block is dead text the user cannot open. So never fence a URL, never put one in inline code, and never answer "here are the links" with a snippet. Label a link with what it is ("OpenClickly on GitHub"), and when you give several, put each on its own bullet with its own link.
+
 A fenced block is never a whole answer on its own. Say in one plain sentence what the code does or when to reach for it, so the reply reads like something a person wrote rather than a bare snippet.
 
 Call a tool when it makes the answer more accurate. Anything time-sensitive or past your cutoff should be checked rather than guessed, and never invent tool output. If a tool fails, say so and answer with what you know.