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.

providers/vercel.py

@@ -1,43 +0,0 @@
-"""Vercel AI Gateway provider profile.
-
-AI Gateway routes to multiple backends. Hermes sends attribution
-headers and full reasoning config passthrough.
-"""
-
-from typing import Any
-
-from providers import register_provider
-from providers.base import ProviderProfile
-
-
-class VercelAIGatewayProfile(ProviderProfile):
-    """Vercel AI Gateway — attribution headers + reasoning passthrough."""
-
-    def build_api_kwargs_extras(
-        self,
-        *,
-        reasoning_config: dict | None = None,
-        supports_reasoning: bool = True,
-        **ctx: Any,
-    ) -> tuple[dict[str, Any], dict[str, Any]]:
-        extra_body: dict[str, Any] = {}
-        if supports_reasoning and reasoning_config is not None:
-            extra_body["reasoning"] = dict(reasoning_config)
-        elif supports_reasoning:
-            extra_body["reasoning"] = {"enabled": True, "effort": "medium"}
-        return extra_body, {}
-
-
-vercel = VercelAIGatewayProfile(
-    name="ai-gateway",
-    aliases=("vercel", "vercel-ai-gateway", "ai_gateway", "aigateway"),
-    env_vars=("AI_GATEWAY_API_KEY",),
-    base_url="https://ai-gateway.vercel.sh/v1",
-    default_headers={
-        "HTTP-Referer": "https://hermes-agent.nousresearch.com",
-        "X-Title": "Hermes Agent",
-    },
-    default_aux_model="google/gemini-3-flash",
-)
-
-register_provider(vercel)