[BUG] Four shipped plugin agents fail YAML frontmatter parsing and load as placeholders or not at all

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Four agent files that ship in this repository have frontmatter that Claude Code 2.1.259 cannot parse:

- `plugins/plugin-dev/agents/agent-creator.md`
- `plugins/plugin-dev/agents/plugin-validator.md`
- `plugins/plugin-dev/agents/skill-reviewer.md`
- `plugins/pr-review-toolkit/agents/code-simplifier.md`

Their `description:` value spans several lines, the continuation lines (``, `Context:`, `user:`, ...) are not indented, and the first line ends in `Examples:`. What happens depends on how the file is loaded:

- **As a plugin** (`claude --plugin-dir plugins/plugin-dev`): the debug log says `Failed to parse YAML frontmatter`, and the agent is registered anyway with the placeholder description `Agent from plugin-dev plugin`. The agent roster handed to the model then reads `plugin-dev:agent-creator: Agent from plugin-dev plugin (Tools: All tools)` with none of the triggering examples, so the model has nothing to trigger on.
- **Copied to `.claude/agents/`**: the log says `YAML frontmatter in failed to parse and was ignored`, and the agent is absent from the roster.

The other 11 agent files under `plugins/*/agents/` load, and the model receives their descriptions verbatim.

Three properties of the current shape are each fatal on their own (measured with single-variable mutants of agent-creator.md): a plain value that ends in `:`; continuation lines that are not indented; and `: ` inside indented continuation lines (`Context: ...`). The only forms that survive are block scalars (`description: |-`) or a quoted string.

The same shape is what `plugins/plugin-dev/skills/agent-development/SKILL.md` (Complete Format, Minimal Agent), its examples and references, and agent-creator's own system prompt teach, so agents generated with the plugin inherit the defect. `validate-agent.sh` never parses YAML, so it passes these files; the extractors proposed in #89404 and #76985 also accept the shape.

### What Should Happen?

The four agents load with their full descriptions and `` blocks on both paths. At minimum, a frontmatter parse failure in a shipped plugin agent should be visible without `--debug` instead of silently registering a placeholder.

### Error Messages/Logs

```shell
# .claude/agents/ path (claude -p --debug)
[ERROR] YAML frontmatter in /.claude/agents/agent-creator.md failed to parse and was ignored: YAML Parse error: Unexpected token
[ERROR] YAML frontmatter in /.claude/agents/plugin-validator.md failed to parse and was ignored: YAML Parse error: Unexpected token
[ERROR] YAML frontmatter in /.claude/agents/skill-reviewer.md failed to parse and was ignored: YAML Parse error: Unexpected token
[ERROR] YAML frontmatter in /.claude/agents/code-simplifier.md failed to parse and was ignored: YAML Parse error: Unexpected token

# plugin path (claude -p --debug --plugin-dir plugins/plugin-dev)
[WARN] Failed to parse YAML frontmatter in /plugins/plugin-dev/agents/agent-creator.md: YAML Parse error: Unexpected token
[WARN] Failed to parse YAML frontmatter in /plugins/plugin-dev/agents/plugin-validator.md: YAML Parse error: Unexpected token
[WARN] Failed to parse YAML frontmatter in /plugins/plugin-dev/agents/skill-reviewer.md: YAML Parse error: Unexpected token
```

### Steps to Reproduce

1. Clone the repository and copy the plugin-dev agents into a fresh project:
```bash
git clone --depth 1 https://github.com/anthropics/claude-code
mkdir -p /tmp/agents-probe/.claude/agents
cp claude-code/plugins/plugin-dev/agents/*.md /tmp/agents-probe/.claude/agents/
```
2. Start a one-turn session there with the debug log on:
```bash
cd /tmp/agents-probe
claude -p --debug --debug-file /tmp/loader.log --max-turns 1 "hi" >/dev/null
grep -c 'failed to parse and was ignored' /tmp/loader.log # prints 3
```
3. Repeat on the plugin path from an empty directory:
```bash
mkdir -p /tmp/empty && cd /tmp/empty
claude -p --debug --debug-file /tmp/plug.log --plugin-dir /path/to/claude-code/plugins/plugin-dev --max-turns 1 "List the agent types available to you, verbatim."
grep 'Failed to parse YAML frontmatter' /tmp/plug.log # 3 lines
```
The model's answer lists the three plugin-dev agents with the placeholder text `Agent from plugin-dev plugin` and no examples.
4. Convert one file's description to `description: |-` with every line indented by two spaces and repeat: the log line disappears and the agent's full description reaches the model.

### Claude Model

Other

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.259 (Claude Code)

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

Non-interactive/CI environment

### Additional Information

- The roster observations above were taken from the request body sent to the model (a local stand-in for the API endpoint), not from the model's own account of itself. Interactive mode was not measured; `claude -p` was.
- The bug is in file loading, before any model is called, so the Claude Model field does not apply.
- The fix is on the branch `fix/agent-description-block-scalars` of konsta95/claude-code: [compare view against main](https://github.com/anthropics/claude-code/compare/main...konsta95:fix/agent-description-block-scalars), [commit e830ff6](https://github.com/konsta95/claude-code/commit/e830ff6701cc867bd01ff0b6ae2d406f8ad59704). It converts the four files to `description: |-` (text unchanged), updates the skill docs and agent-creator's template to teach that form, and changes `validate-agent.sh` to reject the three fatal shapes, with a regression test. Pull request creation is restricted to collaborators on this repository, so a maintainer would need to pull the branch.
- Related: #83803 (origin of #89404), #89404, #76985, #85323, #84427, #66416 all change `validate-agent.sh`; none changes the agent files.

Start by inspecting the four listed agent files and the Complete Format and Minimal Agent sections of plugins/plugin-dev/skills/agent-development/SKILL.md, then run the provided copied-agent and plugin-path reproductions. Compare the behavior with the linked fix branch and regression test; done means all four agents load with their full descriptions and example blocks on both paths, with failures no longer hidden as placeholders.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.