Fix quick-setup doc drift and setup guidance inconsistencies

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `README.md`

## Summary

This PR fixes the text/documentation drift reported in #1767 across quick-setup and setup guidance surfaces.

### Changes

- Removed unsupported `--set-secret` mention from `README.md` quick-setup text and restored the `## License` heading formatting.
- Fixed grammar in `gh-agent-workflows/README.md` quick-setup paragraph ("sets up creates" -> coherent sentence).
- Removed unsupported `--skip-secret` guidance from `gh-agent-workflows/README.md`.
- Updated `scripts/quick-setup.sh` usage comment to remove unsupported `--set-secret` flag.
- Clarified `docs/migration-guide.md` to state that GitHub Agent Workflows do not require the Claude secret and removed contradictory "either" phrasing.
- Reconciled prerequisite guidance by updating `docs/developing.md` to match current setup behavior (Go optional for non-release builds; `curl`/`wget` noted for release binary download).

## Validation

- `make lint`

Closes \#1767.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Text Auditor](https://github.com/elastic/ai-github-actions/actions/runs/31010888194)

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---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** pushSignedCommits: refusing unsigned push for branch 'fix-text-auditor-1767-0fec904d79d61863': Signed-commit payload violates file-protection policy (request_review): README.md, gh-agent-workflows/README.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/31010888194)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 31010888194 -n agent -D /tmp/agent-31010888194

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-31010888194/aw-fix-text-auditor-1767.bundle refs/heads/fix-text-auditor-1767:refs/bundles/create-pr-fix-text-auditor-1767-0fec904d79d61863-e8d4e822
git update-ref refs/heads/fix-text-auditor-1767-0fec904d79d61863 refs/bundles/create-pr-fix-text-auditor-1767-0fec904d79d61863-e8d4e822
git checkout fix-text-auditor-1767-0fec904d79d61863
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-fix-text-auditor-1767-0fec904d79d61863-e8d4e822

# Push the branch to origin
git push origin fix-text-auditor-1767-0fec904d79d61863

# Create the pull request
gh pr create --title 'Fix quick-setup doc drift and setup guidance inconsistencies' --base main --head fix-text-auditor-1767-0fec904d79d61863 --repo elastic/ai-github-actions
```

Review the setup guidance in README.md, gh-agent-workflows/README.md, scripts/quick-setup.sh, docs/migration-guide.md, and docs/developing.md against the current quick-setup behavior. Check the proposed removals and prerequisite wording, then run make lint; done means the documented flags, secret requirements, and setup prerequisites are consistent.

Written by the indexing model from the issue text.