feat(#2500): enrich gsd-codebase-mapper arch-focus ARCHITECTURE.md with ASCII diagrams, data flow traces, and constraints (#2715)

* feat(#2500): enrich gsd-codebase-mapper arch-focus ARCHITECTURE.md template

The codebase mapper's arch-focus template was a sparse structural inventory.
After major refactors, the research/ARCHITECTURE.md (created at /gsd-new-project
and never refreshable) went stale while the refreshable codebase version lacked
the visual richness that makes architecture docs useful for planning.

Add to the ARCHITECTURE.md template:
- <!-- refreshed: {date} --> marker at the top (maintainer request)
- ASCII system overview diagram with component boxes and flow arrows
- Component responsibility table (Component / Responsibility / File)
- Primary request path traces with numbered steps and code references
- Architectural constraints section (threading, global state, circular imports)
- Anti-patterns section with codebase-specific patterns and correct alternatives

All existing sections (Pattern Overview, Layers, Key Abstractions, Entry Points,
Error Handling, Cross-Cutting Concerns) are preserved.

7 new tests in tests/enh-2500-codebase-mapper-arch-rich-format.test.cjs verify
each required section is present in the deployed template.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#2500): resolve CodeRabbit review findings

- Add 'text' language tag to bare ASCII diagram fenced block (markdownlint MD040)
- Tighten data flow test: require '### Primary Request Path' heading, 3+
  numbered steps, and file:line reference pattern — prevents loose-match
  false positives
- Tighten constraints test: require '## Architectural Constraints' heading
  AND Threading / Global state / Circular imports tokens — prevents broad
  keyword matches masking regressions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

agents/gsd-codebase-mapper.md

@@ -339,10 +339,42 @@ Ready for orchestrator summary.
 ## ARCHITECTURE.md Template (arch focus)
 
 ```markdown
+<!-- refreshed: [YYYY-MM-DD] -->
 # Architecture
 
 **Analysis Date:** [YYYY-MM-DD]
 
+## System Overview
+
+```text
+┌─────────────────────────────────────────────────────────────┐
+│                      [Top Layer Name]                        │
+├──────────────────┬──────────────────┬───────────────────────┤
+│   [Component A]  │   [Component B]  │    [Component C]      │
+│  `[path/to/a]`   │  `[path/to/b]`   │   `[path/to/c]`       │
+└────────┬─────────┴────────┬─────────┴──────────┬────────────┘
+         │                  │                     │
+         ▼                  ▼                     ▼
+┌─────────────────────────────────────────────────────────────┐
+│                    [Middle Layer Name]                       │
+│         `[path/to/layer]`                                    │
+└─────────────────────────────────────────────────────────────┘
+         │
+         ▼
+┌─────────────────────────────────────────────────────────────┐
+│  [Store / Output / External]                                 │
+│  `[path/to/store]`                                           │
+└─────────────────────────────────────────────────────────────┘
+```
+
+## Component Responsibilities
+
+| Component | Responsibility | File |
+|-----------|----------------|------|
+| [Name] | [What it owns] | `[path]` |
+| [Name] | [What it owns] | `[path]` |
+| [Name] | [What it owns] | `[path]` |
+
 ## Pattern Overview
 
 **Overall:** [Pattern name]
@@ -363,7 +395,13 @@ Ready for orchestrator summary.
 
 ## Data Flow
 
-**[Flow Name]:**
+### Primary Request Path
+
+1. [Step 1 — entry point] (`[file:line]`)
+2. [Step 2 — processing] (`[file:line]`)
+3. [Step 3 — output/response] (`[file:line]`)
+
+### [Secondary Flow Name]
 
 1. [Step 1]
 2. [Step 2]
@@ -386,6 +424,27 @@ Ready for orchestrator summary.
 - Triggers: [What invokes it]
 - Responsibilities: [What it does]
 
+## Architectural Constraints
+
+- **Threading:** [Threading model — e.g., single-threaded event loop, worker threads used for X]
+- **Global state:** [Any module-level singletons or shared mutable state — list files]
+- **Circular imports:** [Known circular dependency chains, if any]
+- **[Other constraint]:** [Description]
+
+## Anti-Patterns
+
+### [Anti-Pattern Name]
+
+**What happens:** [The incorrect pattern observed in this codebase]
+**Why it's wrong:** [The problem it causes here]
+**Do this instead:** [The correct pattern with file reference]
+
+### [Anti-Pattern Name]
+
+**What happens:** [The incorrect pattern observed in this codebase]
+**Why it's wrong:** [The problem it causes here]
+**Do this instead:** [The correct pattern with file reference]
+
 ## Error Handling
 
 **Strategy:** [Approach]