add Plano agent skills framework and rule set (#797)

* feat: add initial documentation for Plano Agent Skills

* feat: readme with examples

* feat: add detailed skills documentation and examples for Plano

---------

Co-authored-by: Adil Hafeez <adil.hafeez@gmail.com>

skills/rules/routing-default.md

@@ -0,0 +1,70 @@
+---
+title: Always Set Exactly One Default Model Provider
+impact: HIGH
+impactDescription: Without a default provider, Plano has no fallback when routing preferences do not match — requests with unclassified intent will fail
+tags: routing, defaults, model-providers, reliability
+---
+
+## Always Set Exactly One Default Model Provider
+
+When a request does not match any routing preference, Plano forwards it to the `default: true` provider. Without a default, unmatched requests fail. If multiple providers are marked `default: true`, Plano uses the first one — which can produce unexpected behavior.
+
+**Incorrect (no default provider set):**
+
+```yaml
+version: v0.3.0
+
+model_providers:
+  - model: openai/gpt-4o-mini     # No default: true anywhere
+    access_key: $OPENAI_API_KEY
+    routing_preferences:
+      - name: summarization
+        description: Summarizing documents and extracting key points
+
+  - model: openai/gpt-4o
+    access_key: $OPENAI_API_KEY
+    routing_preferences:
+      - name: code_generation
+        description: Writing new functions and implementing algorithms
+```
+
+**Incorrect (multiple defaults — ambiguous):**
+
+```yaml
+model_providers:
+  - model: openai/gpt-4o-mini
+    default: true               # First default
+    access_key: $OPENAI_API_KEY
+
+  - model: openai/gpt-4o
+    default: true               # Second default — confusing
+    access_key: $OPENAI_API_KEY
+```
+
+**Correct (exactly one default, covering unmatched requests):**
+
+```yaml
+version: v0.3.0
+
+model_providers:
+  - model: openai/gpt-4o-mini
+    access_key: $OPENAI_API_KEY
+    default: true               # Handles general/unclassified requests
+    routing_preferences:
+      - name: summarization
+        description: Summarizing documents, articles, and meeting notes
+      - name: classification
+        description: Categorizing inputs, labeling, and intent detection
+
+  - model: openai/gpt-4o
+    access_key: $OPENAI_API_KEY
+    routing_preferences:
+      - name: code_generation
+        description: Writing, debugging, and reviewing code
+      - name: complex_reasoning
+        description: Multi-step math, logical analysis, research synthesis
+```
+
+Choose your most cost-effective capable model as the default — it handles all traffic that doesn't match specialized preferences.
+
+Reference: [https://github.com/katanemo/archgw](https://github.com/katanemo/archgw)