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/refactor-plan.prompt.md

@@ -0,0 +1,66 @@
+---
+mode: 'agent'
+tools: ['codebase', 'terminalCommand']
+description: 'Plan a multi-file refactor with proper sequencing and rollback steps'
+---
+
+# Refactor Plan
+
+Create a detailed plan for this refactoring task.
+
+## Refactor Goal
+
+{{refactor_description}}
+
+## Instructions
+
+1. Search the codebase to understand current state
+2. Identify all affected files and their dependencies
+3. Plan changes in a safe sequence (types first, then implementations, then tests)
+4. Include verification steps between changes
+5. Consider rollback if something fails
+
+## Output Format
+
+```markdown
+## Refactor Plan: [title]
+
+### Current State
+[Brief description of how things work now]
+
+### Target State
+[Brief description of how things will work after]
+
+### Affected Files
+| File | Change Type | Dependencies |
+|------|-------------|--------------|
+| path | modify/create/delete | blocks X, blocked by Y |
+
+### Execution Plan
+
+#### Phase 1: Types and Interfaces
+- [ ] Step 1.1: [action] in `file.ts`
+- [ ] Verify: [how to check it worked]
+
+#### Phase 2: Implementation
+- [ ] Step 2.1: [action] in `file.ts`
+- [ ] Verify: [how to check]
+
+#### Phase 3: Tests
+- [ ] Step 3.1: Update tests in `file.test.ts`
+- [ ] Verify: Run `npm test`
+
+#### Phase 4: Cleanup
+- [ ] Remove deprecated code
+- [ ] Update documentation
+
+### Rollback Plan
+If something fails:
+1. [Step to undo]
+2. [Step to undo]
+
+### Risks
+- [Potential issue and mitigation]
+```
+
+Shall I proceed with Phase 1?