Add Prompt Input to Github Action

[dbpolito]

Bringing part of the idea from #4807

This adds a new input to github action prompt, so we can override it... this opens the possibility to do something like this:

name: opencode-auto-review

on:
  pull_request:
    types: [opened, synchronize, reopened]

jobs:
  review:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pull-requests: write
    steps:
      - uses: actions/checkout@v4
      - uses: sst/opencode/github@latest
        with:
          model: anthropic/claude-sonnet-4-5
          prompt: |
            Review this pull request:
            - Check for code quality issues
            - Look for potential bugs
            - Suggest improvements

We could even ask on opencode github install if they want to install this action too, right?