Add proposal scraper Stop hook example

[WGlynn]

Summary

Adds a second example to examples/hooks/ — a Stop hook that persists proposal blocks (options, alternatives) from assistant responses to a PROPOSALS.md file so they survive session crashes, context compaction, and API errors.

Alongside the existing bash_command_validator_example.py (a PreToolUse hook), this demonstrates a different hook type (Stop) and a different use case (output persistence vs input validation).

Motivation

When an assistant proposes options for a user to choose between — **Option A** / **Option B** / **Option C**, or a numbered decision slate — and the session crashes before the user decides, the options exist only in the chat transcript. Regenerating them in a new session produces different options because LLM sampling is non-deterministic. Persisting proposals the moment they are generated makes the file the source of truth and the chat a view.

Detection

The hook uses two layers of detection:

• Strong signal (auto-trigger): explicit option labels (**Option A**, cycle-style IDs like **C11-A**, or Option A: prose) appearing outside of code spans.
• Weak signal: two or more numbered bold items in combination with a proximity keyword (options, propose, alternative, pick one, which of, choose). This prevents false positives on structured summaries that happen to use numbered bold lists.

Matches inside backtick code spans are stripped before detection so that documentation of the trigger patterns (including this PR description) does not self-trigger.

Behavior

• Reads the session transcript via the transcript_path field from the hook context.
• Extracts the last assistant message text.
• If it looks like a proposal, appends to .claude/PROPOSALS.md (project-local) or falls back to the current working directory.
• Fail-open: any error returns cleanly without blocking the turn.

Testing

The hook was developed iteratively and has been tested against a corpus of real proposal and non-proposal content, including:

• Explicit **Option A** / **Option B** blocks (triggers)
• Cycle-style **C11-A** labels (triggers)
• Numbered bold lists with "options" / "propose" keywords (triggers)
• Completion summaries using numbered bold lists without keywords (does not trigger)
• Documentation mentioning trigger patterns inside backticks (does not trigger)
• Plain questions and answers (does not trigger)

Style

Follows the convention of bash_command_validator_example.py:

• Self-contained, stdlib-only
• Shebang + docstring with settings.json snippet
• Fail-open error handling
• "Change your path to your actual script" note in the docstring

Configuration

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "python3 /path/to/claude-code/examples/hooks/proposal_scraper_example.py"
          }
        ]
      }
    ]
  }
}