fix(automerge): never enqueue into a merge queue as GITHUB_TOKEN

[cmarko89]

Summary

Removes the GITHUB_TOKEN fallback from merge-queue enqueue in reusable-automerge.yml. Enqueue now requires the deploy-bot app. If the app token is missing or the app is refused, the PR is skipped with a warning and retried by the next event or the sweep.

Root cause of the "merge queue hard-wedge"

A merge queue attributes the merge_group event to whoever enqueued the PR. GitHub starts no workflow runs from events caused by GITHUB_TOKEN, so an entry enqueued as github-actions never gets a merge_group CI run, never sees its required ci check, and sits at AWAITING_CHECKS until the timeout.

Evidence, 18 PRs across 3 repos (enqueuing actor versus merge_group runs formed):

Enqueued by	PRs	merge_group runs
a human (cmarko89)	ha-shared-workflows #11 #12 #13, hubspace #23 #24, muninn #159, and #160 #161 #162 after a manual re-enqueue	1 each
github-actions only	ha-shared-workflows #14 #15, hubspace #25 #26 #27 #28, muninn #163 #164 #165	0 each
emkraan-deploy-bot	ha-shared-workflows #16 (verification, 2026-09-23)	1: merged by the queue in 51s

The HA integration repos looked healthy only because their Validate workflow also runs on push with no branch filter. The queue's push of gh-readonly-queue/... is by github-merge-queue[bot] (not suppressed), so that push run reported ci on the group commit. ha-shared-workflows' CI is pull_request + merge_group only, so nothing reported ci and it wedged. muninn #163/#164, documented on 2026-09-19 as a GitHub-side wedge, were the same cause.

The wedged enqueues all came from runs acting as github-actions: callers without app inputs, and workflow_run events, which run main's caller (it lacked the app until today's rollout). All callers now pass the app. This PR makes sure a missing or failed app token can never silently reintroduce the wedge.

Verification

• A mocked run of the script: queue + app enqueues as the app (CLEAN and UNSTABLE-self); queue + no app is skipped loudly with no mutation; queue + app refused is not retried as the token; plain repos keep the token retry.
• Live: docs(readme): add the merge_group trigger to the caller example ha-shared-workflows#16 was enqueued by emkraan-deploy-bot, its merge_group CI run formed 18s later, and the queue merged it 51s after enqueue.