fix(agent): strip tool_use_error tags and render errors in red

When a tool call fails, claude wraps the result text in
<tool_use_error>...</tool_use_error> XML tags. The terminal was
displaying these raw, producing output like:
  '<tool_use_error>File has not been read yet.</tool_use_error>'

Fix renderToolResult in app.js:
- Check c.is_error on the tool_result content block.
- Strip the <tool_use_error>...</tool_use_error> wrapper from the text.
- Render error results with a '✗' prefix under '.tool-result.error'
  (flat, ≤120c) or '.tool-result-block.error' (<details>, longer text).

Add .live .tool-result.error { color: var(--red); } to terminal.css
so error results are visually distinct (red, same as turn-end-fail).

Update terminal-rendering.md row taxonomy to document the two new
error row classes.

Closes #2104.

frontend/packages/shared/src/terminal.css

@@ -120,7 +120,8 @@
 .live .text        { color: var(--fg);    }
 .live .thinking    { color: var(--muted); font-style: italic; }
 .live .tool-use    { color: var(--cyan);  }
-.live .tool-result { color: var(--muted); }
+.live .tool-result         { color: var(--muted); }
+.live .tool-result.error   { color: var(--red);   }
 .live .result      { color: var(--green); }
 .live .note        { color: var(--muted); }
 /* Distinguish stderr lines (orange) and operator-initiated notes