feat(buildrunner): drop per-call queueName, add Factory

[behinddwalls]

Summary

Why?

BuildRunner.Trigger took a queueName argument that selected the
runner-specific job configuration on every call. That put queue routing
on the hot path and on a single verb, leaving Status and Cancel to
rediscover the queue from the build ID. The queue belongs at construction
time, not per call.

What?

• Drop queueName from BuildRunner.Trigger; the verbs speak only in
  builds and changes.
• Add a Factory interface (New(cfg Config) (BuildRunner, error)) and
  a Config struct carrying QueueID. A runner is bound to its Config at
  construction; backends extend Config with their own settings.
• noop: add NewFactory(); keep New().
• Wire the factory end to end: the build and buildsignal controllers hold
  a buildrunner.Factory and build a runner per queue via
  New(Config{QueueID: batch.Queue}). buildsignal loads the batch to
  resolve the queue (TODO: denormalize queue onto the Build). The example
  orchestrator wires buildnoop.NewFactory().
• Docs: BuildRunner is no longer described as a "singleton" but must stay
  safe for concurrent use. Updated the RFC (Construction section) and
  README.

Caching of runners per queue is intentionally omitted for now.

Test Plan

✅ make mocks, make gazelle, make tidy, make fmt
✅ bazel build //...
✅ make test (unit), incl. buildrunner + build + buildsignal