feat(docs+smoke): LB pool live smoke + virtual-llms.md pool semantics (v4 Stage 3)

Smoke (tests/smoke/llm-pool.smoke.test.ts): two in-process registrars
publish virtual Llms with distinct names but a shared poolName, then:

  1. /api/v1/llms/<name>/members surfaces both with the correct
     effective pool key, size, activeCount, and per-member kind/status.
  2. Chat through an agent pinned to one pool member dispatches across
     the pool — verified by running 12 calls and asserting at least
     one response from each backend (the random-shuffle selection
     would have to hit only-A or only-B in 12 fair coin flips, ~1/2048).
  3. Failover: stop one publisher, the surviving member still serves
     chat. /members shows the stopped row as inactive immediately
     (unbindSession runs synchronously on SSE close).

docs/virtual-llms.md gets a full "LB pools (v4)" section with the
two-field schema model, dispatcher selection + failover semantics,
public + virtual declaration examples, list/describe rendering, the
"pin to specific instance" escape hatch, and an API surface entry
for /members. docs/agents.md cross-link extended.

Tests: full smoke 144/144 (was 141, +3 for the new pool smoke).
Stages 1-3 ship the complete v4 — public and virtual Llms can both
join pools, agents transparently load-balance across them, yaml
round-trip preserves poolName, and the existing single-Llm world
keeps working byte-identically when poolName is null.

docs/agents.md

@@ -208,5 +208,8 @@ mcpctl chat reviewer
   extends the same publishing model to **virtual agents** declared in
   mcplocal config — they show up in `mcpctl get agent` with
   `KIND=virtual / STATUS=active` and become chat-able via
-  `mcpctl chat <name>` like any other agent.
+  `mcpctl chat <name>` like any other agent. **v4** adds **pools**:
+  Llms sharing a `poolName` stack into one load-balanced pool that
+  the chat dispatcher transparently widens to at request time, with
+  random selection + sequential failover on transport errors.
 - [chat.md](./chat.md) — `mcpctl chat` flow and LiteLLM-style flags.