docs(observability): mention OrbStack and Colima alongside Docker Desktop

## Summary

Every mention of the Docker requirement in the observability stack refers only to "Docker Desktop":

- `bin/factory-obs:121` — `Install Docker Desktop or the docker-compose v2 plugin.`
- `tools/observability/README.md` quickstart — no runtime alternatives listed
- `skills/factory-obs/SKILL.md` — "Run without Docker — no cloud fallback by design."

On macOS (the most common dev platform for this plugin), two free / lighter alternatives are widely used and work transparently with `docker compose` v2:

- **OrbStack** — native arm64, much lower RAM/CPU footprint than Docker Desktop, no license restrictions for commercial use
- **Colima** — CLI-only, uses Lima + containerd or dockerd, popular with users who don't want a GUI

Both ship a working `docker` CLI and `docker compose` plugin, so `factory-obs up` works unchanged after a one-time `brew install --cask orbstack` (or `brew install colima docker docker-compose && colima start`).

## Suggested changes

1. **`bin/factory-obs:121`** — update the error message:
```
factory-obs: docker compose not found. Install one of: Docker Desktop,
OrbStack (brew install --cask orbstack), or Colima (brew install colima
docker docker-compose && colima start).
```

2. **`tools/observability/README.md`** — add a "Container runtime" section after the quickstart noting supported options, with the OrbStack / Colima install commands.

3. **`skills/factory-obs/SKILL.md`** — replace bare "Docker Desktop" mention with "a `docker compose`–compatible runtime (Docker Desktop, OrbStack, or Colima)".

## Validation

Confirmed working with OrbStack 2.2.1 on macOS arm64 during switchboard-blue onboarding (no other changes required).

## Acceptance criteria

- [ ] Error message in `bin/factory-obs` lists all three supported runtimes
- [ ] `tools/observability/README.md` quickstart documents the alternatives
- [ ] `skills/factory-obs/SKILL.md` no longer says "Docker Desktop" exclusively