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/plano-routing-model-selection/SKILL.md

@@ -0,0 +1,34 @@
+---
+name: plano-routing-model-selection
+description: Optimize Plano model routing and selection. Use for provider defaults, model aliases, passthrough auth, and routing preference quality.
+license: Apache-2.0
+metadata:
+  author: katanemo
+  version: "1.0.0"
+---
+
+# Plano Routing and Model Selection
+
+Use this skill when requests are routed to the wrong model, costs are high, or fallback behavior is unclear.
+
+## When To Use
+
+- "Improve model routing"
+- "Add aliases and defaults"
+- "Fix passthrough auth with proxy providers"
+- "Tune routing preferences for better classification"
+
+## Apply These Rules
+
+- `routing-default`
+- `routing-aliases`
+- `routing-passthrough`
+- `routing-preferences`
+
+## Execution Checklist
+
+1. Ensure exactly one `default: true` provider.
+2. Add semantic aliases for stable client contracts.
+3. Configure passthrough auth only where required.
+4. Rewrite vague preference descriptions with concrete task scopes.
+5. Validate routing behavior using trace-based checks.