манифесты стали данными, заведён convy sync

- убраны комментарии из suite.toml и .conventions.toml: файл, который
  машина переписывает, комментарий через круг не проносит; объяснения
  ушли в README рядом, который suite init теперь заводит
- удалена текстовая правка манифеста целиком — 520 строк ручного
  лексера TOML вместе со всем классом ошибок порчи данных
- запись идёт из структур энкодером; ключ, которого инструмент не
  знает, запись останавливает, а не теряется молча
- convy sync сверяет манифест и подводит под него раскладку файлов:
  чего не хватает — собирает, что осиротело — удаляет, копию с
  локальной частью не трогает никогда

internal/manifest/manifest.go

@@ -1,12 +1,24 @@
-// Package manifest reads suite.toml, the manifest of a conventions suite.
+// Package manifest reads and writes the two manifests of the model.
 //
-// The manifest declares three things: the language the suite's rules are
+// The suite manifest declares three things: the language the suite's rules are
 // written in, its live and retired topics, and its live and retired rule
 // prefixes together with the paths of their files. The tool knows no topic and
 // no prefix in advance — that whole list arrives from here.
+//
+// Both manifests are data the tool edits, so both are decoded into structs and
+// written back out of them. They carry no comments: a file a machine rewrites
+// cannot keep a comment through the round trip, and pretending otherwise costs
+// the comment on a day nobody is watching. What a topic is for is said in the
+// documents next to the manifest, which no command touches.
+//
+// Because a write goes out of the structs, a key the tool does not know would
+// disappear on the next edit. So it does not write at all while one is there:
+// a refusal naming the key is the only outcome that neither loses it nor hides
+// it.
 package manifest
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -38,25 +50,44 @@ const DefaultLanguageCode = "ru"
 // changing — the vocabulary is picked by version and code either way.
 type Language struct {
 	Version     int    `toml:"version"`
-	Lang        string `toml:"lang"`
-	Source      string `toml:"source"`
-	Description string `toml:"description"`
-	Reading     string `toml:"reading"`
+	Lang        string `toml:"lang,omitempty"`
+	Source      string `toml:"source,omitempty"`
+	Description string `toml:"description,omitempty"`
+	Reading     string `toml:"reading,omitempty"`
 }
 
 // Section is a part of the manifest split into a live and a retired half.
 // Retired entries are kept rather than deleted: a topic name and a rule prefix
 // live on in foreign repositories, and neither may ever be reused.
 type Section struct {
-	Live    map[string]string `toml:"live"`
-	Retired map[string]string `toml:"retired"`
+	Live    map[string]string `toml:"live,omitempty"`
+	Retired map[string]string `toml:"retired,omitempty"`
+}
+
+// Add puts an entry into the live half, making the map if there is none.
+func (s *Section) Add(key, value string) {
+	if s.Live == nil {
+		s.Live = make(map[string]string)
+	}
+	s.Live[key] = value
+}
+
+// Retire moves an entry out of the live half into the retired one. A name is
+// never deleted and never reissued: it lives on in foreign repositories, and a
+// name handed out twice starts pointing at something else there.
+func (s *Section) Retire(key, note string) {
+	delete(s.Live, key)
+	if s.Retired == nil {
+		s.Retired = make(map[string]string)
+	}
+	s.Retired[key] = note
 }
 
 // Manifest is a parsed suite.toml.
 type Manifest struct {
 	Language Language `toml:"language"`
-	Topics   Section  `toml:"topics"`
-	Prefixes Section  `toml:"prefixes"`
+	Topics   Section  `toml:"topics,omitempty"`
+	Prefixes Section  `toml:"prefixes,omitempty"`
 
 	// Path is where the manifest was read from.
 	Path string `toml:"-"`
@@ -91,6 +122,33 @@ func Load(root string) (*Manifest, error) {
 	return &m, nil
 }
 
+// Save writes the manifest back to where it was read from.
+func (m *Manifest) Save() error {
+	return save(m.Path, m, m.Undecoded)
+}
+
+// save encodes a manifest and puts it in place.
+func save(path string, value any, undecoded []string) error {
+	if len(undecoded) > 0 {
+		return fmt.Errorf("%s holds %s the tool does not know (%s); a write goes out of what the tool understands, so the key would be dropped — fix the spelling first",
+			path, plural(len(undecoded), "key"), strings.Join(undecoded, ", "))
+	}
+	var b bytes.Buffer
+	enc := toml.NewEncoder(&b)
+	enc.Indent = ""
+	if err := enc.Encode(value); err != nil {
+		return fmt.Errorf("encoding %s: %w", path, err)
+	}
+	return os.WriteFile(path, b.Bytes(), 0o644)
+}
+
+func plural(n int, noun string) string {
+	if n == 1 {
+		return fmt.Sprintf("%d %s", n, noun)
+	}
+	return fmt.Sprintf("%d %ss", n, noun)
+}
+
 // Find walks up from start looking for a directory that holds a suite
 // manifest, so that `convy suite check` works from any subdirectory of a suite.
 func Find(start string) (string, error) {