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/what-context-needed.prompt.md

@@ -0,0 +1,40 @@
+---
+mode: 'agent'
+tools: ['codebase']
+description: 'Ask Copilot what files it needs to see before answering a question'
+---
+
+# What Context Do You Need?
+
+Before answering my question, tell me what files you need to see.
+
+## My Question
+
+{{question}}
+
+## Instructions
+
+1. Based on my question, list the files you would need to examine
+2. Explain why each file is relevant
+3. Note any files you've already seen in this conversation
+4. Identify what you're uncertain about
+
+## Output Format
+
+```markdown
+## Files I Need
+
+### Must See (required for accurate answer)
+- `path/to/file.ts` — [why needed]
+
+### Should See (helpful for complete answer)
+- `path/to/file.ts` — [why helpful]
+
+### Already Have
+- `path/to/file.ts` — [from earlier in conversation]
+
+### Uncertainties
+- [What I'm not sure about without seeing the code]
+```
+
+After I provide these files, I'll ask my question again.