feat(cli+docs): mcpctl get agent KIND/STATUS columns + virtual-agent smoke + docs (v3 Stage 4)

CLI: `mcpctl get agent` table view gains KIND and STATUS columns
mirroring the `get llm` shape from v1. Public agents render as
`public/active` (the AgentRow defaults) and virtual ones surface their
true lifecycle state, so `mcpctl get agent` becomes a single-pane view
for both manually-created and mcplocal-published personas.

Smoke: tests/smoke/virtual-agent.smoke.test.ts mirrors virtual-llm's
in-process registrar pattern — publishes a fake provider + agent in
one round-trip, confirms mcpd surfaces the agent kind=virtual /
status=active under /api/v1/agents, then disconnects and verifies the
paired Llm-and-Agent both flip to inactive (deletion is GC-driven, not
disconnect-driven, so the rows must still exist post-stop). Heartbeat-
stale and 4 h sweep paths are covered by the unit suite to keep smoke
duration in check.

Docs: docs/virtual-llms.md gets a "Virtual agents (v3)" section with a
config sample, lifecycle notes, listing example, and the cluster-wide
name-uniqueness caveat. The API surface block now mentions the new
`agents[]` field on _provider-register, the join-by-session heartbeat
behavior, and the `GET /api/v1/agents` lifecycle fields. docs/agents.md
gains a one-paragraph note pointing to the v3 publishing path.

Tests: full smoke suite 141/141 (was 139, +2 new), unit suites
unchanged (mcpd 860/860, mcplocal 723/723).

docs/agents.md

@@ -204,5 +204,9 @@ mcpctl chat reviewer
 - [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.
+  publishing mcplocal — mcpd never holds the local URL or key. **v3**
+  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.
 - [chat.md](./chat.md) — `mcpctl chat` flow and LiteLLM-style flags.