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.

docs/terminal-rendering.md

@@ -63,6 +63,8 @@ parent's negative pull.
 | `.tool-result` (flat) | `←  <txt>` | muted | short `tool_result` (≤120c, non-recv) | stream-json |
 | `.tool-result-block` `<details>` | `Nl · headline` | muted, body is text | long generic `tool_result` | stream-json |
 | `.tool-result-block` `<details open>` | `recv ← <txt>` | muted, body is markdown | `tool_result` correlated to a prior `recv` tool_use via id | stream-json |
+| `.tool-result.error` (flat) | `✗  <msg>` | red | `tool_result` with `is_error: true` (≤120c); `<tool_use_error>` wrapper stripped | stream-json |
+| `.tool-result-block.error` `<details>` | `Nl · headline` | red, body is text | long error `tool_result` (`is_error: true`); wrapper stripped | stream-json |
 | `.tool-use` | `⌁  task <id> started · <desc> [type]` | cyan | claude Task-tool subagent start (dead path — `Task` omitted from agent allow-list) | `renderTaskEvent` |
 | `.turn-end-ok` / `.turn-end-fail` / `.tool-result` | `⌁  task <id> ✓/✗/◌ <status> · <desc> · → <output_file>` | green / red / muted | claude Task-tool result (dead path for agents) | `renderTaskEvent` |
 | `.note` | `· <text>` | muted | harness chatter | `LiveEvent::Note` |