feat: overhaul context length detection with models.dev and provider-aware resolution (#2158)

Replace the fragile hardcoded context length system with a multi-source
resolution chain that correctly identifies context windows per provider.

Key changes:

- New agent/models_dev.py: Fetches and caches the models.dev registry
  (3800+ models across 100+ providers with per-provider context windows).
  In-memory cache (1hr TTL) + disk cache for cold starts.

- Rewritten get_model_context_length() resolution chain:
  0. Config override (model.context_length)
  1. Custom providers per-model context_length
  2. Persistent disk cache
  3. Endpoint /models (local servers)
  4. Anthropic /v1/models API (max_input_tokens, API-key only)
  5. OpenRouter live API (existing, unchanged)
  6. Nous suffix-match via OpenRouter (dot/dash normalization)
  7. models.dev registry lookup (provider-aware)
  8. Thin hardcoded defaults (broad family patterns)
  9. 128K fallback (was 2M)

- Provider-aware context: same model now correctly resolves to different
  context windows per provider (e.g. claude-opus-4.6: 1M on Anthropic,
  128K on GitHub Copilot). Provider name flows through ContextCompressor.

- DEFAULT_CONTEXT_LENGTHS shrunk from 80+ entries to ~16 broad patterns.
  models.dev replaces the per-model hardcoding.

- CONTEXT_PROBE_TIERS changed from [2M, 1M, 512K, 200K, 128K, 64K, 32K]
  to [128K, 64K, 32K, 16K, 8K]. Unknown models no longer start at 2M.

- hermes model: prompts for context_length when configuring custom
  endpoints. Supports shorthand (32k, 128K). Saved to custom_providers
  per-model config.

- custom_providers schema extended with optional models dict for
  per-model context_length (backward compatible).

- Nous Portal: suffix-matches bare IDs (claude-opus-4-6) against
  OpenRouter's prefixed IDs (anthropic/claude-opus-4.6) with dot/dash
  normalization. Handles all 15 current Nous models.

- Anthropic direct: queries /v1/models for max_input_tokens. Only works
  with regular API keys (sk-ant-api*), not OAuth tokens. Falls through
  to models.dev for OAuth users.

Tests: 5574 passed (18 new tests for models_dev + updated probe tiers)
Docs: Updated configuration.md context length section, AGENTS.md

Co-authored-by: Test <test@test.com>

website/docs/user-guide/configuration.md

@@ -416,7 +416,19 @@ LLM_MODEL=meta-llama/Llama-3.1-70B-Instruct-Turbo
 
 ### Context Length Detection
 
-Hermes automatically detects your model's context length by querying the endpoint's `/v1/models` response. For most setups this works out of the box. If detection fails (the model name doesn't match, the endpoint doesn't expose `/v1/models`, etc.), Hermes falls back to a high default and probes downward on context-length errors.
+Hermes uses a multi-source resolution chain to detect the correct context window for your model and provider:
+
+1. **Config override** — `model.context_length` in config.yaml (highest priority)
+2. **Custom provider per-model** — `custom_providers[].models.<id>.context_length`
+3. **Persistent cache** — previously discovered values (survives restarts)
+4. **Endpoint `/models`** — queries your server's API (local/custom endpoints)
+5. **Anthropic `/v1/models`** — queries Anthropic's API for `max_input_tokens` (API-key users only)
+6. **OpenRouter API** — live model metadata from OpenRouter
+7. **Nous Portal** — suffix-matches Nous model IDs against OpenRouter metadata
+8. **[models.dev](https://models.dev)** — community-maintained registry with provider-specific context lengths for 3800+ models across 100+ providers
+9. **Fallback defaults** — broad model family patterns (128K default)
+
+For most setups this works out of the box. The system is provider-aware — the same model can have different context limits depending on who serves it (e.g., `claude-opus-4.6` is 1M on Anthropic direct but 128K on GitHub Copilot).
 
 To set the context length explicitly, add `context_length` to your model config:
 
@@ -427,10 +439,23 @@ model:
   context_length: 131072  # tokens
 ```
 
-This takes highest priority — it overrides auto-detection, cached values, and hardcoded defaults.
+For custom endpoints, you can also set context length per model:
+
+```yaml
+custom_providers:
+  - name: "My Local LLM"
+    base_url: "http://localhost:11434/v1"
+    models:
+      qwen3.5:27b:
+        context_length: 32768
+      deepseek-r1:70b:
+        context_length: 65536
+```
+
+`hermes model` will prompt for context length when configuring a custom endpoint. Leave it blank for auto-detection.
 
 :::tip When to set this manually
-- Your model shows "2M context" in the status bar (detection failed)
+- You're using Ollama with a custom `num_ctx` that's lower than the model's maximum
 - You want to limit context below the model's maximum (e.g., 8k on a 128k model to save VRAM)
 - You're running behind a proxy that doesn't expose `/v1/models`
 :::