test(budget): add DesignDocs tests; replace PLAN-137 with clean design doc

- budget/budget_test.go: add TestFit_DesignDocsInSystemPrompt,
  TestFit_DesignDocsTrimmedBeforeFileContext, TestFit_DesignDocsEmptyNoHeading
  to cover the new DesignDocs section through Fit() and buildResult()
- Remove PLAN-137.md (contained raw thinking stream, not suitable as repo doc)
- Add docs/DESIGN-137-doc-map.md with clean architectural decision record

budget/budget_test.go

@@ -200,3 +200,72 @@ func TestFit_NeverExceedsLimit(t *testing.T) {
 		t.Errorf("EstTokens %d exceeds limit %d (trimmed: %v)", result.EstTokens, limit, result.Trimmed)
 	}
 }
+
+// TestFit_DesignDocsInSystemPrompt verifies that DesignDocs content appears in the
+// system prompt under the expected heading.
+func TestFit_DesignDocsInSystemPrompt(t *testing.T) {
+	s := Sections{
+		SystemBase: "base instructions",
+		DesignDocs: "# Foo Design\n\nSome design content.",
+		Diff:       "diff content",
+		UserMeta:   "PR meta",
+	}
+	result := Fit("gpt-4.1", s)
+
+	if !strings.Contains(result.SystemPrompt, "## Design Documents") {
+		t.Errorf("expected ## Design Documents heading in system prompt, got:\n%s", result.SystemPrompt)
+	}
+	if !strings.Contains(result.SystemPrompt, "# Foo Design") {
+		t.Errorf("expected design doc content in system prompt, got:\n%s", result.SystemPrompt)
+	}
+	// Sanity: design docs should NOT appear in user prompt.
+	if strings.Contains(result.UserPrompt, "## Design Documents") {
+		t.Errorf("design docs heading should not be in user prompt, got:\n%s", result.UserPrompt)
+	}
+}
+
+// TestFit_DesignDocsTrimmedBeforeFileContext verifies trim ordering:
+// DesignDocs is trimmed (third) before FileContext (fourth), after Conventions.
+func TestFit_DesignDocsTrimmedBeforeFileContext(t *testing.T) {
+	// Fill budget so design docs and file context can't both fit.
+	// gpt-4.1 limit = 128_000 - 4_000 = 124_000 tokens.
+	// SystemBase = 480_000 bytes ≈ 120_000 tokens → leaves ~4_000 tokens.
+	// Diff = 8_000 bytes ≈ 2_000 tokens.
+	// DesignDocs = 20_000 bytes ≈ 5_000 tokens → exceeds remaining 2_000.
+	// Expected: DesignDocs trimmed; FileContext (very small) survives.
+	s := Sections{
+		SystemBase:  strings.Repeat("s", 480_000),
+		DesignDocs:  strings.Repeat("d", 20_000),
+		FileContext: "important_file_context",
+		Diff:        strings.Repeat("x", 8_000),
+		UserMeta:    "PR meta",
+	}
+	result := Fit("gpt-4.1", s)
+
+	found := false
+	for _, item := range result.Trimmed {
+		if strings.HasPrefix(item, "design docs") {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("expected 'design docs' in trimmed list, got: %v", result.Trimmed)
+	}
+}
+
+// TestFit_DesignDocsEmptyNoHeading verifies that an empty DesignDocs field
+// does not inject the ## Design Documents heading into the system prompt.
+func TestFit_DesignDocsEmptyNoHeading(t *testing.T) {
+	s := Sections{
+		SystemBase: "base",
+		DesignDocs: "",
+		Diff:       "diff",
+		UserMeta:   "meta",
+	}
+	result := Fit("gpt-4.1", s)
+
+	if strings.Contains(result.SystemPrompt, "## Design Documents") {
+		t.Errorf("empty DesignDocs should not inject heading, got:\n%s", result.SystemPrompt)
+	}
+}