slash-commands: remove commit-status gate, document GitHub App flow

The merge gate is now solely the App's APPROVE review satisfying branch
protection's required-reviews rule, plus auto-merge. The commit-status
mechanism is removed entirely:

- delete slash-commands/status sub-action
- drop status-context input and setCommitStatus() from main.js
- drop the now-unused hadApproved/hadHold label snapshot

README rewritten to document the GitHub App setup (MCP_COMMANDER_APP_ID
+ MCP_COMMANDER_APP_KEY secrets, actions/create-github-app-token to mint
the installation token) as the primary flow. The App token is passed as
both github-token (team checks) and bot-token (approval/auto-merge) so
approvals come from the App identity and always count toward required
reviews, independent of the "Allow GitHub Actions to approve" repo
setting.

README.md

@@ -10,4 +10,3 @@ GitHub Actions relevant to the management of MCP repositories.
 | [`cloudflare-pages-preview/cleanup`](cloudflare-pages-preview/README.md) | Delete Cloudflare Pages deployments for a closed PR's branch alias and update the sticky comment |
 | [`hugo-build`](hugo-build/README.md) | Install Go + Hugo extended and build a Hugo site (preview or production mode) |
 | [`slash-commands`](slash-commands/README.md) | Handle `/lgtm` (core-maintainers only: label + approve + auto-merge), `/hold`, and `/stageblog` PR slash commands; invalidate approval on push |
-| [`slash-commands/status`](slash-commands/README.md#slash-commandsstatus-sub-action) | _Optional:_ set a pending/failure commit status for callers using status-check-based gating instead of required-reviews + auto-merge |