feat(providers): make all 33 providers pluggable under plugins/model-providers/

Every provider profile is now a self-contained plugin under
plugins/model-providers/<name>/, mirroring the plugins/platforms/
pattern established for IRC and Teams. The ProviderProfile ABC
stays in providers/; the per-provider profile data moves out.

- plugins/model-providers/<name>/__init__.py calls register_provider()
- plugins/model-providers/<name>/plugin.yaml declares kind: model-provider
- providers/__init__.py._discover_providers() lazily scans bundled plugins
  then $HERMES_HOME/plugins/model-providers/<name>/ (user override path)
- User plugins with the same name override bundled ones (last-writer-wins
  in register_provider)
- Legacy providers/<name>.py layout still supported for back-compat with
  out-of-tree editable installs
- Hermes PluginManager: new kind=model-provider; skipped like memory
  plugins (providers/ discovery owns them); standalone plugins with
  register_provider+ProviderProfile in their __init__.py auto-coerce to
  this kind (same heuristic as memory providers)
- skip_names extended to include 'model-providers' so the general
  PluginManager doesn't double-scan the category
- 4 new tests in tests/providers/test_plugin_discovery.py covering
  bundled discovery, user override, and general-loader isolation
- Docs updated: website/docs/developer-guide/adding-providers.md,
  provider-runtime.md, providers/README.md, plugins/model-providers/README.md

No API break: auth.py / config.py / doctor.py / models.py / runtime_provider.py /
model_metadata.py / auxiliary_client.py / chat_completions.py / run_agent.py
all still consume providers via get_provider_profile() / list_providers() —
they just now see plugin-discovered entries instead of pkgutil-iterated ones.

Third parties can now drop a single directory into
~/.hermes/plugins/model-providers/<name>/ to add or override an inference
provider without touching the repo.

plugins/model-providers/nous/__init__.py

@@ -0,0 +1,53 @@
+"""Nous Portal provider profile."""
+
+from typing import Any
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+
+class NousProfile(ProviderProfile):
+    """Nous Portal — product tags, reasoning with Nous-specific omission."""
+
+    def build_extra_body(
+        self, *, session_id: str | None = None, **context
+    ) -> dict[str, Any]:
+        return {"tags": ["product=hermes-agent"]}
+
+    def build_api_kwargs_extras(
+        self,
+        *,
+        reasoning_config: dict | None = None,
+        supports_reasoning: bool = False,
+        **context,
+    ) -> tuple[dict[str, Any], dict[str, Any]]:
+        """Nous: passes full reasoning_config, but OMITS when disabled."""
+        extra_body = {}
+        if supports_reasoning:
+            if reasoning_config is not None:
+                rc = dict(reasoning_config)
+                if rc.get("enabled") is False:
+                    pass  # Nous omits reasoning when disabled
+                else:
+                    extra_body["reasoning"] = rc
+            else:
+                extra_body["reasoning"] = {"enabled": True, "effort": "medium"}
+        return extra_body, {}
+
+
+nous = NousProfile(
+    name="nous",
+    aliases=("nous-portal", "nousresearch"),
+    env_vars=("NOUS_API_KEY",),
+    display_name="Nous Research",
+    description="Nous Research — Hermes model family",
+    signup_url="https://nousresearch.com/",
+    fallback_models=(
+        "hermes-3-405b",
+        "hermes-3-70b",
+    ),
+    base_url="https://inference.nousresearch.com/v1",
+    auth_type="oauth_device_code",
+)
+
+register_provider(nous)