Add optional Langfuse tracing to Cursor PR review action

[Codex]

Cursor agent runs lacked Langfuse observability; the action now ships optional tracing using the cursor-langfuse hooks when Langfuse keys are provided.

• Inputs & wiring: Added optional langfuse-secret-key, langfuse-public-key, langfuse-base-url inputs; forwarded to the cursor provider.
• Hooks bundling: Vendorized cursor-langfuse hook assets under config/langfuse-hooks and install them with npm ci when enabled.
• Provider behavior: Cursor provider now copies hooks into .cursor, sets LANGFUSE_*, and keeps prior behavior when keys are absent.
• Docs: Documented the tracing toggle and inputs in the action README.

Example usage enabling tracing:

- uses: ./.github/actions/agentic-pr-review
  with:
    app-id: ${{ secrets.APP_ID }}
    private-key: ${{ secrets.APP_PRIVATE_KEY }}
    provider-api-key: ${{ secrets.CURSOR_API_KEY }}
    pull-request-number: ${{ github.event.pull_request.number }}
    langfuse-secret-key: ${{ secrets.LANGFUSE_SECRET_KEY }}
    langfuse-public-key: ${{ secrets.LANGFUSE_PUBLIC_KEY }}
    # optional: langfuse-base-url: https://cloud.langfuse.com