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.

slash-commands/action.yml

@@ -74,14 +74,6 @@ inputs:
     description: Merge method for auto-merge — one of squash, merge, rebase
     required: false
     default: squash
-  status-context:
-    description: >-
-      Commit-status context name written after each label mutation. Set to
-      empty string to disable status writes (recommended when using
-      enable-auto-merge + required reviews as the merge gate instead). If set,
-      must match the status sub-action's status-context.
-    required: false
-    default: slash-commands/approval
   stageblog-workflow:
     description: >-
       Workflow file name (e.g. stage-blog.yml) to dispatch when /stageblog is
@@ -124,7 +116,6 @@ runs:
         SUBMIT_REVIEW: ${{ inputs.submit-review }}
         ENABLE_AUTO_MERGE: ${{ inputs.enable-auto-merge }}
         AUTO_MERGE_METHOD: ${{ inputs.auto-merge-method }}
-        STATUS_CONTEXT: ${{ inputs.status-context }}
         STAGEBLOG_WORKFLOW: ${{ inputs.stageblog-workflow }}
         STAGEBLOG_PATHS: ${{ inputs.stageblog-paths }}
         ORG_TOKEN: ${{ inputs.github-token }}