Add context-engineering collection

Add tools for maximizing GitHub Copilot effectiveness through better
context management:

- Instructions: Guidelines for structuring code so Copilot understands it
- Agent: Context Architect - plans multi-file changes by mapping dependencies
- Prompts:
  - context-map: Map all affected files before changes
  - what-context-needed: Ask Copilot what files it needs
  - refactor-plan: Create phased refactor plans with rollback steps

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

instructions/context-engineering.instructions.md

@@ -0,0 +1,43 @@
+---
+description: 'Guidelines for structuring code and projects to maximize GitHub Copilot effectiveness through better context management'
+---
+
+# Context Engineering
+
+Principles for helping GitHub Copilot understand your codebase and provide better suggestions.
+
+## Project Structure
+
+- **Use descriptive file paths**: `src/auth/middleware.ts` > `src/utils/m.ts`. Copilot uses paths to infer intent.
+- **Colocate related code**: Keep components, tests, types, and hooks together. One search pattern should find everything related.
+- **Export public APIs from index files**: What's exported is the contract; what's not is internal. This helps Copilot understand boundaries.
+
+## Code Patterns
+
+- **Prefer explicit types over inference**: Type annotations are context. `function getUser(id: string): Promise<User>` tells Copilot more than `function getUser(id)`.
+- **Use semantic names**: `activeAdultUsers` > `x`. Self-documenting code is AI-readable code.
+- **Define constants**: `MAX_RETRY_ATTEMPTS = 3` > magic number `3`. Named values carry meaning.
+
+## Working with Copilot
+
+- **Keep relevant files open in tabs**: Copilot uses open tabs as context signals. Working on auth? Open auth-related files.
+- **Position cursor intentionally**: Copilot prioritizes code near your cursor. Put cursor where context matters.
+- **Use Copilot Chat for complex tasks**: Inline completions have minimal context. Chat mode sees more files.
+
+## Context Hints
+
+- **Add a COPILOT.md file**: Document architecture decisions, patterns, and conventions Copilot should follow.
+- **Use strategic comments**: At the top of complex modules, briefly describe the flow or purpose.
+- **Reference patterns explicitly**: "Follow the same pattern as `src/api/users.ts`" gives Copilot a concrete example.
+
+## Multi-File Changes
+
+- **Describe scope first**: Tell Copilot all files involved before asking for changes. "I need to update the User model, API endpoint, and tests."
+- **Work incrementally**: One file at a time, verifying each change. Don't ask for everything at once.
+- **Check understanding**: Ask "What files would you need to see?" before complex refactors.
+
+## When Copilot Struggles
+
+- **Missing context**: Open the relevant files in tabs, or explicitly paste code snippets.
+- **Stale suggestions**: Copilot may not see recent changes. Re-open files or restart the session.
+- **Generic answers**: Be more specific. Add constraints, mention frameworks, reference existing code.