Define and codify pattern evaluation criteria

## What
Formalise Decision Confidence as the evaluation standard for all patterns (new and existing). Document the principle, assessment output format, and gatekeeping pattern rule in the green-software-pattern skill. Create a reusable template the team can use when reviewing patterns.

## Why
The team currently lacks shared language for assessing patterns, leading to circular debate (service mesh pattern took 15 minutes with no decision). Decision Confidence provides objective criteria that replace subjective judgment and make assessment faster and consistent.

## Success
- Decision Confidence principle is documented in the skill
- Assessment Output template is added to the skill with clear examples
- Gatekeeping pattern rule (concrete conditions, not vague language) is codified
- Team has reviewed and agreed on the criteria
- Service mesh pattern assessment demonstrates the framework in action

## SoW

* [x] Document Decision Confidence core principle in skill
* [x] Add Assessment Output template with sections (Passes On / Fails On / To Publish, Requires)
* [ ] Add examples showing how to apply the template
* [x] Document gatekeeping pattern rule (no vague conditions)
* [x] Get team buy-in via GitHub consensus motion
* [ ] Publish service mesh pattern assessment as proof of concept