Architecture skill with Mermaid pattern documentation

[brooke-hamilton]

Description

Adds the architecture-documenter skill to the Radius Copilot CLI with comprehensive Mermaid diagram patterns and examples.

The prompt used to create this skill used the /create-skill prompt that is currently bundled with VS Code Insiders. Here is what that prompt was given:

/create-skill Create a skill in the radius project that is very good at documenting application architectures. 
- It is an expert at abstracting the application architecture and generating understandable mermaid diagrams. 
- When asked it can go much deeper into the detailed interactions. It is especially good at producing true-to-code sequence diagrams (mermaid) and component diagrams that show entity relationships.
- It can distill ccomplex code concepts into simple to understand human readable text.
- It can dive deep into existing code, especially golang, to figure out how things work.
- It can suggest archtitectural improvements that would simplify the code.
- It can answer questions about the existing architecture

A follow-up prompt was useful in filling out the High Level Components section:

validate the "High-Level Components" section. 
- Are those paths and names correct? If not, fix them.
- Are there more paths and names that should be added?

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Contributor checklist

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Not applicable

Changes

• Added arch-documenter skill with SKILL.md documentation
• Added Mermaid pattern references for architecture diagrams

[Copilot]

Pull request overview

Adds a new arch-documenter Copilot skill under .github/skills/ to help generate code-grounded architecture documentation, including reusable Mermaid diagram templates.

Changes:

• Added arch-documenter skill definition and guidance (SKILL.md)
• Added a Mermaid diagram patterns reference document with reusable templates

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
.github/skills/arch-documenter/SKILL.md	Defines the skill’s purpose, procedure, Radius-specific context, and a recommended output format.
.github/skills/arch-documenter/references/mermaid-patterns.md	Provides reusable Mermaid diagram templates and diagramming tips.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.98%. Comparing base (209adb7) to head (ce8c9fa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11281   +/-   ##
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  Coverage   50.98%   50.98%           
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  Files         679      679           
  Lines       43174    43174           
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+ Hits        22012    22013    +1     
+ Misses      19040    19038    -2     
- Partials     2122     2123    +1     

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