fix(demo): let demo-queue inherit the fake build runner

## Summary

### Why?

`demo-queue` pinned the GitHub Actions build runner, so every land waited on a real CI run. That is not what the demo is for: it exists to show the queue batching, speculating and merging, and a walkthrough that spends most of its time watching a workflow spin says very little about any of that.

The configuration had also drifted from its own documentation, which already describes the fake runner as the default and real CI as the thing you opt into.

### What?

The queue no longer names a build runner, so it inherits `{type: fake}` from the defaults and every build succeeds instantly. A land now completes in seconds.

The comment that offered the Actions block said to "replace the line above with the block below", which after this points at the change provider rather than at anything to do with builds. It now says to add the block, and explains what inheriting the default actually gets you.

Real CI remains one uncommented block away, and the how-to still documents the three things it needs.

## Test Plan

✅ `make local-provider-start PROVIDER=github` parses the configuration on startup and refuses to start on an invalid one, so a malformed profile fails loudly rather than silently falling back.

✅ No code change: the fake runner is an existing implementation already used by every other queue in this file and by the local provider.

Author: behinddwalls

service/submitqueue/demo/provider/github/profiles.yaml

@@ -17,20 +17,13 @@ queues:
     # Enterprise, add `github: {baseUrl: https://ghe.example.com/api/v3}`.
     changeProvider: {type: github}
 
-
-    # Every build succeeds instantly, so a land completes in seconds and the
-    # demo exercises the merge rather than waiting on CI.
-    buildRunner: 
-      type: githubactions
-      owner: behinddwalls
-      repo: sq-demo
-      workflow: ci.yml        # file name or numeric workflow id
-      ref: main               # the branch the workflow definition is read from
+    # The build runner is inherited from the defaults above, so every build
+    # succeeds instantly. A land then completes in seconds, and the demo shows
+    # the queue and the merge rather than spending its time waiting on CI.
     #
-    # To run real CI instead, replace the line above with the block below. It
-    # needs a workflow in the target repository that is triggerable by
-    # workflow_dispatch and accepts the sq_base_uris / sq_head_uris inputs, and
-    # a token with the `workflow` scope.
+    # To run real CI instead, add the block below. It needs a workflow in the
+    # target repository that is triggerable by workflow_dispatch and accepts the
+    # sq_base_uris / sq_head_uris inputs, and a token with the `workflow` scope.
     #
     # buildRunner:
     #   type: githubactions