document the auto-merge arming window in the README

[bborbe]

Adds a README section explaining that auto-merge arming uses the GraphQL enablePullRequestAutoMerge mutation (no REST endpoint exists — the plausible-looking PUT returns 404), that it is rejected with UNPROCESSABLE: Pull request is in clean status on an already-mergeable PR, and why a fast-turnaround PR can therefore go unarmed without anything being broken.

Docs only.

This PR also serves as the live end-to-end test of the v0.5.2 arming path: it carries the auto-merge label and is deliberately left without a review trigger, so it stays blocked long enough for the watcher's poll to arm it.

[ben-s-pull-request-reviewer]

{
"verdict": "approve",
"summary": "PR adds a new 'Auto-merge arming window' section to the README explaining the GraphQL-only nature of GitHub's auto-merge, the UNPROCESSABLE rejection for already-mergeable PRs, and the cursor-based polling behavior that gives roughly one arming attempt per PR update. All factual claims were cross-checked against pkg/githubclient.go and found to be accurate. CHANGELOG.md entry correctly reflects the change. No issues found.",
"comments": [],
"concerns_addressed": [
"correctness: README states no REST endpoint exists, confirmed by pkg/githubclient.go:203-205 which explicitly documents that PUT /repos/{owner}/{repo}/pulls/{number}/auto-merge returns 404 Not Found",
"correctness: README states the exact UNPROCESSABLE error message, confirmed by pkg/githubclient.go:213 and the error construction at lines 253-259",
"correctness: README describes cursor-based polling with one arming attempt per PR update, matching the search-based architecture in pkg/githubclient.go:121-169"
]
}