feat(resolvemcp): add model descriptions, usage guide and catalogue export

- modelregistry: ModelInfo (description, purpose, tags, column docs) with
  external JSON loader, Describer fallback and gorm/bun/comment tag support
- resolvemcp: surface descriptions in list_tables and describe_table, send a
  usage guide as MCP server instructions, add package docs
- add BuildCatalog/ExportCatalog to write a JSON or Markdown API catalogue
- document the descriptions map and catalogue in the README

pkg/modelregistry/info_test.go

@@ -0,0 +1,68 @@
+package modelregistry
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type infoModel struct {
+	ID    int    `json:"id" gorm:"primaryKey;comment:Row id"`
+	Email string `json:"email" bun:"email,comment:Login address"`
+	Name  string `json:"name" note:"Display name"`
+	Plain string `json:"plain"`
+}
+
+func (infoModel) ModelDescription() string { return " From the model " }
+
+func TestFieldComment(t *testing.T) {
+	typ := reflect.TypeOf(infoModel{})
+	want := map[string]string{"ID": "Row id", "Email": "Login address", "Name": "Display name", "Plain": ""}
+	for field, exp := range want {
+		sf, _ := typ.FieldByName(field)
+		if got := FieldComment(sf); got != exp {
+			t.Errorf("%s = %q, want %q", field, got, exp)
+		}
+	}
+}
+
+func TestModelInfoPrecedence(t *testing.T) {
+	r := NewModelRegistry()
+	if err := r.RegisterModel("public.items", infoModel{}); err != nil {
+		t.Fatal(err)
+	}
+	if got := r.ResolveModelInfo("public.items").Description; got != "From the model" {
+		t.Errorf("describer fallback = %q", got)
+	}
+
+	n, err := r.LoadModelInfo(strings.NewReader(
+		`{"public.items":{"description":"From file","tags":["a"],"columns":{"email":"Mail"}},"public.later":{"purpose":"p"}}`))
+	if err != nil || n != 2 {
+		t.Fatalf("load n=%d err=%v", n, err)
+	}
+	info := r.ResolveModelInfo("public.items")
+	if info.Description != "From file" || info.Columns["email"] != "Mail" || len(info.Tags) != 1 {
+		t.Errorf("file info = %+v", info)
+	}
+	if _, ok := r.GetModelInfo("public.later"); !ok {
+		t.Error("info for a not-yet-registered model must be kept")
+	}
+
+	// returned info is a copy
+	info.Columns["email"] = "changed"
+	if got, _ := r.GetModelInfo("public.items"); got.Columns["email"] != "Mail" {
+		t.Error("GetModelInfo leaked internal map")
+	}
+}
+
+func TestLoadModelInfoRejectsBadInput(t *testing.T) {
+	r := NewModelRegistry()
+	for _, in := range []string{`not json`, `{"a":{"descripton":"typo"}}`} {
+		if _, err := r.LoadModelInfo(strings.NewReader(in)); err == nil {
+			t.Errorf("expected error for %q", in)
+		}
+	}
+	if _, err := r.LoadModelInfoFile("/nonexistent/x.json"); err == nil {
+		t.Error("expected error for missing file")
+	}
+}