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/anthropic/__init__.py

@@ -0,0 +1,52 @@
+"""Native Anthropic provider profile."""
+
+import json
+import logging
+import urllib.request
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+logger = logging.getLogger(__name__)
+
+
+class AnthropicProfile(ProviderProfile):
+    """Native Anthropic — uses x-api-key header, not Bearer."""
+
+    def fetch_models(
+        self,
+        *,
+        api_key: str | None = None,
+        timeout: float = 8.0,
+    ) -> list[str] | None:
+        """Anthropic uses x-api-key header and anthropic-version."""
+        if not api_key:
+            return None
+        try:
+            req = urllib.request.Request("https://api.anthropic.com/v1/models")
+            req.add_header("x-api-key", api_key)
+            req.add_header("anthropic-version", "2023-06-01")
+            req.add_header("Accept", "application/json")
+            with urllib.request.urlopen(req, timeout=timeout) as resp:
+                data = json.loads(resp.read().decode())
+            return [
+                m["id"]
+                for m in data.get("data", [])
+                if isinstance(m, dict) and "id" in m
+            ]
+        except Exception as exc:
+            logger.debug("fetch_models(anthropic): %s", exc)
+            return None
+
+
+anthropic = AnthropicProfile(
+    name="anthropic",
+    aliases=("claude", "claude-oauth", "claude-code"),
+    api_mode="anthropic_messages",
+    env_vars=("ANTHROPIC_API_KEY", "ANTHROPIC_TOKEN", "CLAUDE_CODE_OAUTH_TOKEN"),
+    base_url="https://api.anthropic.com",
+    auth_type="api_key",
+    default_aux_model="claude-haiku-4-5-20251001",
+)
+
+register_provider(anthropic)

plugins/model-providers/anthropic/plugin.yaml

@@ -0,0 +1,5 @@
+name: anthropic-provider
+kind: model-provider
+version: 1.0.0
+description: Anthropic (Claude)
+author: Nous Research