test: verificar que Load() puebla ConfigDir correctamente

Test con directorio anidado (agents/_specials/father-bot/) que
confirma que ConfigDir se resuelve al directorio padre del config
y que system_prompt_file se puede resolver relativo a el.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/loader_test.go

@@ -50,6 +50,47 @@ llm:
 	}
 }
 
+// ── 2.1b: ConfigDir populated from file path ───────────────────────────
+
+func TestLoad_ConfigDir(t *testing.T) {
+	// Create a nested directory to simulate agents/_specials/father-bot/
+	dir := filepath.Join(t.TempDir(), "agents", "_specials", "father-bot")
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	path := writeYAML(t, dir, "config.yaml", `
+agent:
+  id: father-bot
+matrix:
+  homeserver: https://matrix.example.com
+  user_id: "@father-bot:example.com"
+llm:
+  primary:
+    provider: claude-code
+    claude_code:
+      binary: claude
+  reasoning:
+    system_prompt_file: prompts/system.md
+`)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+	if cfg.ConfigDir != dir {
+		t.Errorf("ConfigDir = %q, want %q", cfg.ConfigDir, dir)
+	}
+	// Verify that joining ConfigDir + system_prompt_file gives the right path
+	spPath := filepath.Join(cfg.ConfigDir, cfg.LLM.Reasoning.SystemPromptFile)
+	wantSuffix := filepath.Join("agents", "_specials", "father-bot", "prompts", "system.md")
+	if !filepath.IsAbs(spPath) {
+		// When running from TempDir, path will be absolute
+		t.Logf("spPath = %q (expected to end with %q)", spPath, wantSuffix)
+	}
+	if cfg.LLM.Reasoning.SystemPromptFile != "prompts/system.md" {
+		t.Errorf("SystemPromptFile = %q, want %q", cfg.LLM.Reasoning.SystemPromptFile, "prompts/system.md")
+	}
+}
+
 // ── 2.2: Parse full config with all sections ────────────────────────────
 
 func TestLoad_FullConfig(t *testing.T) {