architecture: add context engineering guide

Author: OgeonX-Ai

docs/architecture/context-engineering.md

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+# Context Engineering
+
+Context engineering is the discipline of giving the agent the right information at the right time.
+
+## Context engineering loop
+
+```mermaid
+flowchart TD
+    A[Goal] --> B[Find relevant context]
+    B --> C[Filter noise]
+    C --> D[Summarize constraints]
+    D --> E[Select loop]
+    E --> F[Verify assumptions]
+```
+
+## Context layers
+
+1. Repository purpose.
+2. Current task.
+3. Project instructions.
+4. Architecture docs.
+5. Relevant source files.
+6. Tests and verifiers.
+7. Recent failures or project memory.
+
+## Rules
+
+- Load less context when the task is narrow.
+- Load more context when risk or uncertainty is high.
+- Prefer repository evidence over memory.
+- Keep assumptions explicit.