feat: Update github-copilot-cli-expert skill with latest CLI docs (#2122)

* feat: Update github-copilot-cli-expert skill with latest CLI docs

Replaces outdated README.md (based on old gh copilot extension) with
comprehensive SKILL.md + reference.md + examples.md structure for the
new standalone copilot CLI.

Changes:
- SKILL.md: Quick reference with installation, commands, shortcuts
- reference.md: Complete command reference, config, security model
- examples.md: 24 practical workflow examples

Fixes #2121

* fix: Correct typo 'gadgugi' -> 'gadugi' in USER_PREFERENCES.md

Fixed typo in gadugi-agentic-test repo name across all 5 copies of
USER_PREFERENCES.md

* feat: Enhance github-copilot-cli-expert skill with prerequisites and version note

- Add 'Before First Use' section with requirements and authentication
- Add 'First-Time Setup Issues' troubleshooting section
- Add version note recommending /help for current syntax
- Sync all changes to 4 locations

Addresses reviewer feedback from PR #2122
References: #2121

---------

Co-authored-by: Ubuntu <azureuser@amplihack-dev.ftnmxvem3frujn3lepas045p5c.xx.internal.cloudapp.net>

Author: rysweet

.claude/context/USER_PREFERENCES.md

@@ -196,15 +196,15 @@ Example: "Always run tests before committing"
 
 **Mandatory End-to-End Testing for Every PR**
 
-I always want you to test each PR like a user would, from the outside in, not just unit testing. For instance you should use "uvx --from git..." syntax to test the branch. You can use agentic test scenarios defined with github.com/rysweet/gadgugi-agentic-test or your own auto mode to test features.
+I always want you to test each PR like a user would, from the outside in, not just unit testing. For instance you should use "uvx --from git..." syntax to test the branch. You can use agentic test scenarios defined with github.com/rysweet/gadugi-agentic-test or your own auto mode to test features.
 
 **Implementation Requirements:**
 
 - MUST test with `uvx --from git+https://github.com/org/repo@branch-name package command`
 - MUST verify the actual user workflow that was broken/enhanced
 - MUST validate error messages, configuration updates, and user experience
 - MUST document test results showing the fix works in realistic conditions
-- Can use gadgugi-agentic-test framework for complex test scenarios
+- Can use gadugi-agentic-test framework for complex test scenarios
 - Can use auto mode for automated feature testing
 
 **This is MANDATORY for Step 8 (Mandatory Local Testing) in DEFAULT_WORKFLOW.md**