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>

prompts/context-map.prompt.md

@@ -0,0 +1,53 @@
+---
+mode: 'agent'
+tools: ['codebase']
+description: 'Generate a map of all files relevant to a task before making changes'
+---
+
+# Context Map
+
+Before implementing any changes, analyze the codebase and create a context map.
+
+## Task
+
+{{task_description}}
+
+## Instructions
+
+1. Search the codebase for files related to this task
+2. Identify direct dependencies (imports/exports)
+3. Find related tests
+4. Look for similar patterns in existing code
+
+## Output Format
+
+```markdown
+## Context Map
+
+### Files to Modify
+| File | Purpose | Changes Needed |
+|------|---------|----------------|
+| path/to/file | description | what changes |
+
+### Dependencies (may need updates)
+| File | Relationship |
+|------|--------------|
+| path/to/dep | imports X from modified file |
+
+### Test Files
+| Test | Coverage |
+|------|----------|
+| path/to/test | tests affected functionality |
+
+### Reference Patterns
+| File | Pattern |
+|------|---------|
+| path/to/similar | example to follow |
+
+### Risk Assessment
+- [ ] Breaking changes to public API
+- [ ] Database migrations needed
+- [ ] Configuration changes required
+```
+
+Do not proceed with implementation until this map is reviewed.