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/custom/__init__.py

@@ -0,0 +1,68 @@
+"""Custom / Ollama (local) provider profile.
+
+Covers any endpoint registered as provider="custom", including local
+Ollama instances. Key quirks:
+  - ollama_num_ctx → extra_body.options.num_ctx (local context window)
+  - reasoning_config disabled → extra_body.think = False
+"""
+
+from typing import Any
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+
+class CustomProfile(ProviderProfile):
+    """Custom/Ollama local provider — think=false and num_ctx support."""
+
+    def build_api_kwargs_extras(
+        self,
+        *,
+        reasoning_config: dict | None = None,
+        ollama_num_ctx: int | None = None,
+        **ctx: Any,
+    ) -> tuple[dict[str, Any], dict[str, Any]]:
+        extra_body: dict[str, Any] = {}
+
+        # Ollama context window
+        if ollama_num_ctx:
+            options = extra_body.get("options", {})
+            options["num_ctx"] = ollama_num_ctx
+            extra_body["options"] = options
+
+        # Disable thinking when reasoning is turned off
+        if reasoning_config and isinstance(reasoning_config, dict):
+            _effort = (reasoning_config.get("effort") or "").strip().lower()
+            _enabled = reasoning_config.get("enabled", True)
+            if _effort == "none" or _enabled is False:
+                extra_body["think"] = False
+
+        return extra_body, {}
+
+    def fetch_models(
+        self,
+        *,
+        api_key: str | None = None,
+        timeout: float = 8.0,
+    ) -> list[str] | None:
+        """Custom/Ollama: base_url is user-configured; fetch if set."""
+        if not self.base_url:
+            return None
+        return super().fetch_models(api_key=api_key, timeout=timeout)
+
+
+custom = CustomProfile(
+    name="custom",
+    aliases=(
+        "ollama",
+        "local",
+        "vllm",
+        "llamacpp",
+        "llama.cpp",
+        "llama-cpp",
+    ),
+    env_vars=(),  # No fixed key — custom endpoint
+    base_url="",  # User-configured
+)
+
+register_provider(custom)