feat(ai): model profiles, capability discovery, and agentless /ai list|models

Make connecting any model a config step, not a code change:
- models.toml named profiles (api_key_env names an env var, never the key)
- providers gain available_models(); add preflight + --list-models/--check
- /ai list and /ai models in-room; client probes local Ollama for
  /ai models when no agent is running, and /ai list hints to summon one
- docs/providers.md provider guide + examples/echo_provider.py
- README: command table, AI section, layout updated

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

examples/echo_provider.py

@@ -0,0 +1,34 @@
+"""Minimal bring-your-own Provider example.
+
+A Provider just turns (system prompt + messages) into one reply string. Anything
+with ``name``, ``model`` and a ``complete()`` method qualifies — no base class,
+no SDK. Point the agent at it with the ``module:Class`` spec:
+
+    python -m cmd_chat.agent 127.0.0.1 3000 --no-tls --password hunter2 \
+        --provider examples.echo_provider:EchoProvider
+
+or via models.toml:
+
+    [echo]
+    provider = "examples.echo_provider:EchoProvider"
+    model    = "echo-1"
+
+Implementing ``available_models()`` is optional; it powers ``--list-models``,
+``--check`` preflight, and the in-room ``/ai models`` command.
+"""
+
+from __future__ import annotations
+
+
+class EchoProvider:
+    name = "echo"
+
+    def __init__(self, model: str = "echo-1"):
+        self.model = model
+
+    def complete(self, system: str, messages: list) -> str:
+        last = messages[-1].content if messages else ""
+        return f"echo: {last}"
+
+    def available_models(self) -> list[str]:  # optional
+        return ["echo-1"]