docs: Add integrations and prompt inputs to README

[Copilot]

integrations and prompt inputs were implemented in action.yml but missing from the README.

Changes

• issue_body: Fixed typo ("he body" → "The body"), corrected default value to match action.yml, and added $RUN_URL and $PROMPT as documented template variables
• integrations: New section documenting supported values (doc-sentinel, promptless, dosu, claude, opencode, copilot, cursor), warning behavior for invalid values, and the special copilot behavior (auto-assigns issue rather than appending to accordion)
• prompt: New section documenting the default prompt text, how it maps to integration mentions (e.g. @claude <prompt>), and its availability as $PROMPT in issue_body

- uses: doc-detective/github-action@v1
  with:
    create_issue_on_fail: true
    integrations: claude,copilot
    prompt: Investigate the Doc Detective test failures and suggest fixes for the documentation.

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[Copilot]

Pull request overview

This PR adds documentation to the README for the integrations and prompt inputs that were already implemented in action.yml but not previously documented. It also fixes a typo and corrects the default value shown for issue_body.

Changes:

• Fixed typo ("he body" → "The body") and updated the issue_body default to match action.yml, adding $RUN_URL and $PROMPT as documented template variables
• Added new integrations section documenting supported values, invalid value behavior, and the special copilot behavior
• Added new prompt section documenting the default text, integration usage, and $PROMPT availability in issue_body

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[promptless]

📝 Documentation updates detected!

New suggestion: Add $PROMPT variable to issue_body description in CI docs

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