[Presets] Update system_prompt for router usage

src/dstack/_internal/cli/services/presets/resources/system_prompt.md

@@ -260,9 +260,9 @@ mindful of which specific change was the root cause.
 {"resources": {...}, "context_length": ..., "benchmark": {...}, "learned": ..., "failed": ...}
 ```
 
-2. In case the task is using node groups (e.g. for PD disaggregation),
-   instead of a single `resources` it includes `groups`, and the fields are
-   these and no others:
+2. In case the task is using node groups (a router group plus worker
+   groups, see `## Fleet Topology`), instead of a single `resources` it
+   includes `groups`, and the fields are these and no others:
 
 ```
 {"groups": [[{...}], [{...}, {...}], [{...}]], "context_length": ..., "benchmark": {...}, "learned": ..., "failed": ...}
@@ -321,9 +321,9 @@ serving engine: use <!--?if dataset-->`dataset` and `concurrency`<!--?else-->`co
 `shared_prefix_tokens`<!--?end--> from `constraints.json` and measure all trials the same
 way so that their results are comparable with each other.
 
-In case the task is using PD disaggregation, run benchmarks via SSH inside
-the router node, directly against the router engine. Never benchmark prefill
-or decode workers — each handles only part of a request.
+In case the task is using a router (see `## Fleet Topology`), run benchmarks
+via SSH inside the router node, directly against the router, never against a
+worker, so that results stay comparable across trials.
 
 <!--?if dataset-->
 Before any benchmark, reset the serving engine's prefix cache, or restart the
@@ -506,6 +506,32 @@ Use these offers when selecting fleet, backend, and hardware.
 To classify each backend's capabilities, fetch `https://dstack.ai/docs/concepts/backends.md` and classify from the fetched document, not from memory. VM-based backends are listed under `## VM-based` (they support idle instances and instance volumes). Kubernetes backend is listed under `## Container-based`, but supports instance volumes and thus is preferred over other container-based backends.
 SSH fleets can be treated as VM-based backends as they support both idle instances (its equivalent) and instance volumes.
 
+## Fleet Topology
+
+If a fleet has `placement: cluster` and a CPU-only instance, that instance
+must host a router (see `https://dstack.ai/docs/concepts/services/#router`),
+and every trial and the final service run behind it. The GPU instances run
+the serving engine as workers in one of two forms: aggregated, where each
+worker handles both prefill and decode, or PD disaggregated, where the
+workers are split into prefill and decode groups. Use `router: sglang`
+(Shepherd Model Gateway, SMG) and connect the workers over gRPC, not HTTP.
+Report the topology decision via `progress` before trial 1.
+
+Why: with an SMG router, request parsing, serialization, and tokenization
+move from the serving engine to the router, and workers only process already
+tokenized inputs, so latency improves just by introducing the router. With
+multiple workers, the router improves performance in two ways.
+
+First, cache-aware routing tracks each worker's KV-cache state and sends a
+request to the worker that already holds its prefix.
+
+Second, load modeling predicts how long a request will take to complete on
+each worker and the router's `least_load` policy sends the request to the
+worker predicted to finish it soonest.
+
+Connecting the workers over gRPC is required for SMG's cache-aware routing
+and load modeling.
+
 # Final Service
 
 Once the trials are over, pick the best trial that has not been verified yet
@@ -565,9 +591,9 @@ trial benchmarks so that the results are comparable with each other. Attach to
 the service with `dstack attach <run name>`, which enables `ssh <run name>`
 into the replica.
 
-In case the service is using PD disaggregation, run the final benchmark
-inside the router replica, directly against the router engine. Attach to it
-via `dstack attach <run name> --replica <replica num> --job <job num>`.
+In case the service is using a router (see `## Fleet Topology`), run the
+final benchmark inside the router replica, directly against the router.
+Attach to it via `dstack attach <run name> --replica <replica num> --job <job num>`.
 
 If the service or its benchmark cannot be completed, stop that service,
 pick the next-best trial, and repeat, until a service is verified or there