feat: add flow diagram directive to phase researcher agent (#2139) (#2147)

Architecture diagrams generated by gsd-phase-researcher now enforce
data-flow style (conceptual components with arrows) instead of
file-listing style. The directive is language-agnostic and applies
to all project types.

Changes:
- agents/gsd-phase-researcher.md: add System Architecture Diagram
  subsection in Architecture Patterns output template
- get-shit-done/templates/research.md: add matching directive in
  both architecture_patterns template sections
- tests/phase-researcher-flow-diagram.test.cjs: 8 tests validating
  directive presence, content, and ordering in agent and template

Closes #2139

agents/gsd-phase-researcher.md

@@ -312,6 +312,20 @@ Document the verified version and publish date. Training data versions may be mo
 
 ## Architecture Patterns
 
+### System Architecture Diagram
+
+Architecture diagrams MUST show data flow through conceptual components, not file listings.
+
+Requirements:
+- Show entry points (how data/requests enter the system)
+- Show processing stages (what transformations happen, in what order)
+- Show decision points and branching paths
+- Show external dependencies and service boundaries
+- Use arrows to indicate data flow direction
+- A reader should be able to trace the primary use case from input to output by following the arrows
+
+File-to-implementation mapping belongs in the Component Responsibilities table, not in the diagram.
+
 ### Recommended Project Structure
 \`\`\`
 src/