YardMaster: a team gateway and setup UI for switchyard-server

[bricke]

Hi! I built YardMaster, a self-hosted gateway that lets a
team share one switchyard-server.

• Per-person tokens. Each person has their own ym_ token, and the provider keys stay on the
  server. The gateway strips the token and sets x-switchyard-origin to the person. It also scopes
  x-switchyard-session-id to that person, so routing affinity never mixes two people.
• Routing setup without editing TOML. A wizard covers providers, models and routes, with every
  strategy explained. Each change is checked with switchyard-server --dry-run, shown as a diff,
  and applied with a graceful restart. If /health doesn't come back, it rolls back automatically.
• Usage by person, route, model and tier, built from the routing log. This includes how much
  traffic the router kept off the expensive models and what the judge cost. It stores metadata
  only, never prompts.

It ships as one Docker image (ghcr.io/bricke/yardmaster) that builds Switchyard v0.3.0 from the
release tag.

I wrote up what I learned running it on real traffic (judge choice, reasoning settings per
provider) in docs/tested.md.
Feedback is welcome, especially on whether I'm relying on anything that's likely to change before
v1.0. I've also opened a PR adding a short integration page: #989.