feat: embed file content in message template via {path} placeholder

[Copilot]

Wrapping file output in a code block previously required piping through $GITHUB_ENV with a heredoc. This adds support for using both path and message together, embedding file content via a {path} placeholder.

Changes

• src/config.ts: When path and message are both provided and message contains {path}, file content is substituted at every occurrence. Falls back to existing behavior (file content used directly) when no placeholder is present.
• action.yml: Removed prefix/suffix inputs; updated message and path descriptions to document the placeholder.
• README.md: Added usage example; updated input reference docs.
• __tests__/config.test.ts: Tests for single-file embed, multi-occurrence replacement, multi-file glob, and no-placeholder fallback.

Usage

- name: Run Test
  run: rake test > result.txt

- uses: marocchino/sticky-pull-request-comment@v3
  with:
    path: result.txt
    message: |
      ```
      {path}
      ```

{path} is intentionally brace-style (not $path) — safe from shell expansion and unlikely to appear in normal message text.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Wrap file output with triple backticks (`)</issue_title>
<issue_description>What's the simplest way to take the output of a file and wrap it with a prefix/suffix like ``` so it ends up like:

    ```
    <file.txt output>
    ```
```</issue_description>

<agent_instructions>이거좀 구현해봐 줄레?
탬플릿 인젝션같은 시큐리티 이슈는 없었으면 좋겠어.
먼저 아이디어를 이야기해주면 어떻게 구현할지 그중에서 정하는 방식으로 하자.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Wrap file output with triple backticks (`) #1577

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.