Add VictoriaMetrics / Grafana Mimir support to the Prometheus backend

VictoriaMetrics and Grafana Mimir both speak the Prometheus query API, but with small differences (URL base path and multi-tenancy headers such as `AccountID`/`X-Scope-OrgID`). Today the agent's Prometheus backend assumes vanilla Prometheus, so users running VM or Mimir can't point the agent at them cleanly.

### Goal
Make the existing Prometheus observability backend work against VictoriaMetrics and Mimir, either via config knobs on the existing client or a thin backend variant.

### Scope
- Support a configurable base path / URL prefix (e.g. VM's `/select/0/prometheus`, Mimir's `/prometheus`).
- Support an optional tenant header (`X-Scope-OrgID` for Mimir, `AccountID`/tenant for VM).
- Surface these as `values.yaml` config on the runner.
- Unit test covering the path + header difference using `httptest.Server`.
- Add a `values.yaml` example and a short docs note.

### Pointers
- `runner/pkg/observability/prometheus/`
- `runner/docs/development.md` → "Adding a new action"
- `runner/docs/configuration.md` for the env-var surface

### Acceptance
- `make test` passes with new coverage for the tenant-header / base-path handling.
- A documented config example lets a user target VM or Mimir without code changes.

Start in runner/pkg/observability/prometheus/ and review the existing backend configuration, then read runner/docs/configuration.md for the env-var surface. Add the requested values.yaml configuration and unit coverage with httptest.Server for base paths and tenant headers, plus the documented example and docs note. Run make test; done means VM or Mimir can be targeted without code changes.

Written by the indexing model from the issue text.