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

CHANGELOG.md

@@ -9,6 +9,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ### Added
 
 - **`@gsd-build/sdk` — Phase 1 typed query foundation** — Registry-based `gsd-sdk query` command, classified errors (`GSDQueryError`), and unit-tested handlers under `sdk/src/query/` (state, roadmap, phase lifecycle, init, config, validation, and related domains). Implements incremental SDK-first migration scope approved in #2083; builds on validated work from #2007 / `feat/sdk-foundation` without migrating workflows or removing `gsd-tools.cjs` in this phase.
+- **Flow diagram directive for phase researcher** — `gsd-phase-researcher` now enforces data-flow architecture diagrams instead of file-listing diagrams. Language-agnostic directive added to agent prompt and research template. (#2139)
 
 ## [1.35.0] - 2026-04-10