feat: virtual-LLM smoke test + docs (v1 Stage 6)

Final stage of v1.

Smoke (mcplocal/tests/smoke/virtual-llm.smoke.test.ts):
- Spins an in-process LlmProvider that returns canned content.
- Runs the registrar against the live mcpd in fulldeploy.
- Asserts: row appears with kind=virtual / status=active, infer
  through /api/v1/llms/<name>/infer comes back through the SSE
  relay with the provider's content + finish_reason, and a 503
  appears immediately after registrar.stop() (publisher offline).
- Times out / cleanup paths idempotent so re-runs against the same
  cluster don't litter rows. The 90-s heartbeat-stale flip and 4-h
  GC are unit-tested — too slow for smoke.

Docs:
- New docs/virtual-llms.md: when to use this vs creating a regular
  Llm row, how to opt-in via publish: true, the lifecycle table,
  the inference-relay sequence, the v1 streaming caveat, the v2-v5
  roadmap, and the full /api/v1/llms/_provider-* surface.
- agents.md cross-links virtual-llms.md alongside personalities/chat.
- README's Agents section gains a "Virtual LLMs" subsection.

Workspace suite: 2043/2043 (smoke files run separately). v1 closes.

Stage roadmap (each its own future PR):
  v2 wake-on-demand · v3 virtual agents · v4 LB pool · v5 task queue

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/agents.md

@@ -201,4 +201,8 @@ mcpctl chat reviewer
 - [personalities.md](./personalities.md) — named overlays of prompts on
   top of an agent. Same agent, different prompt bundles, picked per-turn
   via `--personality <name>` or `agent.defaultPersonality`.
+- [virtual-llms.md](./virtual-llms.md) — local LLMs (e.g. `vllm-local`)
+  publishing themselves into `mcpctl get llm` so anyone can chat with
+  them via `mcpctl chat-llm <name>`. Inference is relayed through the
+  publishing mcplocal — mcpd never holds the local URL or key.
 - [chat.md](./chat.md) — `mcpctl chat` flow and LiteLLM-style flags.