chore: mesh upgrade, clean up legacy special case code, simplify model selection for mesh (#5289)

Shared compute now has exactly two model choices: MeshLLM's virtual
`mesh`
model, or a model you name. Buzz picks between them in one place, and
buzz-agent no longer knows meshes exist.

## What changed

- **MeshLLM v0.74.0 → v0.75.1.** v0.75.0 added
`degrade_to_single_model`, so a
  `model=mesh` request is answered by one served model when there is no
committee to form, instead of failing. v0.75.1 adds Mesh-LLM#1196, which
skips stale pre-0.75 runtime cache entries rather than aborting startup
on
them — without it, anyone who had run mesh on 0.73/0.74 could not start.
- **Deleted the client-side mesh catalog probe.** buzz-agent used to
poll
`/v1/models` (5s TTL, 30s cooldown, two-observation debounce) to decide
whether `mesh` was safe to send. MeshLLM now decides per request, so the
  polling, its hysteresis, and its 503 fallback are gone.
- **One mapping point.** `relay_mesh_wire_model()` turns the stored
value into
a wire name: `auto` becomes `mesh`, a named model passes through. The
spawn
env, the ACP harness, and the readiness probe all use it, so they cannot
disagree — previously `BUZZ_ACP_MODEL` and the probe both said `auto`, a
name
  the mesh does not advertise.
- **Removed the `nostr-relay-pool` advisory exception.** #5404 allowed
RUSTSEC-2026-0243 "after mesh-llm migrates to nostr-sdk >= 0.45".
v0.75.1
does, so the retired crate is gone from both lockfiles and the exception
  would only mask a future advisory for it.
- **Deleted `scripts/ensure-mesh-native-runtime.sh`** and its six
justfile call
sites. It built llama.cpp from source into the runtime cache; the app
already
  downloads the signed release runtime itself, and CI never called it.

## Why it is better

**−639 lines of Rust.** Availability is decided by the node that knows
the
answer, per request, instead of by a client cache that could be stale
for up to
30 seconds. A second worker joining now takes effect on the next request
rather
than after two confirming probes.

## Behaviour change

A 503 on an explicit `mesh` request takes the ordinary transport retry
under
the same model instead of failing over to a second one — there is no
second
model to fail over to now. MoA repairs partial committee results
internally
before it reaches that point.

## Validation

`crates/buzz-relay/examples/mesh_agent_e2e.rs` now sends `mesh` where it
previously sent `auto` or the physical model id, so no leg was covering
what
Buzz actually puts on the wire. 4/4 on gemma-4-E4B, gemma-4-26B-A4B, and
Qwen3-8B — including a real ACP tool call through `mesh` into
buzz-dev-mcp,
asserted by reading the written file back off disk.

Hand-tested in the desktop app on both gemma-4 sizes: picked Auto, agent
logged
`model_id=mesh`, replied in channel.

## Not covered

A committee that forms and then loses a worker returns 502, and that
needs two
workers to reproduce — not testable on one machine.

---------

Signed-off-by: Michael Neale <michael.neale@gmail.com>
Co-authored-by: Michael Neale <michael.neale@gmail.com>

scripts/ensure-mesh-native-runtime.sh

@@ -1,48 +0,0 @@
-#!/usr/bin/env bash
-set -euo pipefail
-
-ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
-BACKEND="${1:-}"
-if [[ -z "$BACKEND" ]]; then
-  case "$(uname -s)" in
-    Darwin) BACKEND="metal" ;;
-    *) BACKEND="cpu" ;;
-  esac
-fi
-
-CACHE_DIR="${MESH_LLM_NATIVE_RUNTIME_CACHE_DIR:-$ROOT/.cache/mesh-llm-native-runtime}"
-OUT_DIR="${MESH_LLM_NATIVE_RUNTIME_OUT_DIR:-$ROOT/.cache/mesh-llm-native-runtime-artifacts}"
-
-metadata="$($ROOT/bin/cargo metadata --manifest-path "$ROOT/desktop/src-tauri/Cargo.toml" --features mesh-llm --format-version 1)"
-SDK_MANIFEST="$(python3 -c 'import json,sys; data=json.load(sys.stdin); print(next(p["manifest_path"] for p in data["packages"] if p["name"]=="mesh-llm-sdk"))' <<<"$metadata")"
-MESH_ROOT="$(cd "$(dirname "$SDK_MANIFEST")/../.." && pwd)"
-MESH_VERSION="$(python3 -c 'import json,sys; data=json.load(sys.stdin); print(next(p["version"] for p in data["packages"] if p["name"]=="mesh-llm-sdk"))' <<<"$metadata")"
-
-case "$(uname -s)/$(uname -m)/$BACKEND" in
-  Darwin/arm64/metal) RUNTIME_ID="meshllm-native-runtime-darwin-aarch64-metal" ;;
-  Darwin/x86_64/metal) RUNTIME_ID="meshllm-native-runtime-darwin-x86_64-metal" ;;
-  */*/cpu)
-    case "$(uname -s)/$(uname -m)" in
-      Darwin/arm64) RUNTIME_ID="meshllm-native-runtime-darwin-aarch64-cpu" ;;
-      Darwin/x86_64) RUNTIME_ID="meshllm-native-runtime-darwin-x86_64-cpu" ;;
-      Linux/x86_64) RUNTIME_ID="meshllm-native-runtime-linux-x86_64-cpu" ;;
-      Linux/aarch64) RUNTIME_ID="meshllm-native-runtime-linux-aarch64-cpu" ;;
-      *) RUNTIME_ID="" ;;
-    esac
-    ;;
-  *) RUNTIME_ID="" ;;
-esac
-
-if [[ -n "$RUNTIME_ID" && -f "$CACHE_DIR/$MESH_VERSION/$RUNTIME_ID/manifest.json" ]]; then
-  printf '%s\n' "$CACHE_DIR"
-  exit 0
-fi
-
-echo "Preparing MeshLLM native runtime ($BACKEND) for MeshLLM $MESH_VERSION..." >&2
-runtime_dir="$(cd "$MESH_ROOT" && scripts/ci-prepare-native-runtime.sh "$OUT_DIR" "$BACKEND")"
-version="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1]))["runtime"].get("mesh_version") or "unknown")' "$runtime_dir/manifest.json")"
-id="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1]))["runtime"]["id"])' "$runtime_dir/manifest.json")"
-mkdir -p "$CACHE_DIR/$version"
-rm -rf "$CACHE_DIR/$version/$id"
-cp -a "$runtime_dir" "$CACHE_DIR/$version/$id"
-printf '%s\n' "$CACHE_DIR"