feat: derive recommended version from changelog; no version file required

- RecommendedTag now reads the current version from the most recent
  released section heading in the changelog (## [x.y.z] - ...) when no
  --version-file flag is given, removing the dependency on a separate
  version file for recommendation.
- When the changelog contains no prior releases, the base version
  defaults to 0.0.0, so the first recommended tag is v1.0.0 (or higher
  depending on unreleased content).
- Prepare creates the release-version file if it does not already exist,
  so new repositories do not need to pre-seed it.
- Add tests covering changelog-based version resolution, first-release
  default, and automatic file creation.
- Update README and changelog unreleased section to document the new
  behaviour.

internal/vociferate/vociferate.go

@@ -15,6 +15,8 @@ const (
 	defaultChangelog   = "changelog.md"
 )
 
+var releasedSectionRe = regexp.MustCompile(`(?m)^## \[(\d+\.\d+\.\d+)\] - `)
+
 type Options struct {
 	VersionFile    string
 	VersionPattern string
@@ -66,9 +68,22 @@ func RecommendedTag(rootDir string, options Options) (string, error) {
 		return "", err
 	}
 
-	currentVersion, err := readCurrentVersion(rootDir, resolved)
-	if err != nil {
-		return "", err
+	var currentVersion string
+	if options.VersionFile != "" {
+		currentVersion, err = readCurrentVersion(rootDir, resolved)
+		if err != nil {
+			return "", err
+		}
+	} else {
+		version, found, err := readLatestChangelogVersion(rootDir, resolved.Changelog)
+		if err != nil {
+			return "", err
+		}
+		if !found {
+			currentVersion = "0.0.0"
+		} else {
+			currentVersion = version
+		}
 	}
 
 	unreleasedBody, err := readUnreleasedBody(rootDir, resolved.Changelog)
@@ -146,6 +161,9 @@ func updateVersionFile(rootDir, version string, options resolvedOptions) error {
 	path := filepath.Join(rootDir, options.VersionFile)
 	contents, err := os.ReadFile(path)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return os.WriteFile(path, []byte(version+"\n"), 0o644)
+		}
 		return fmt.Errorf("read version file: %w", err)
 	}
 
@@ -244,6 +262,20 @@ func readChangelogState(rootDir, changelogPath string) (string, string, int, int
 	return unreleasedBody, text, afterHeader, nextSectionStart, path, nil
 }
 
+func readLatestChangelogVersion(rootDir, changelogPath string) (string, bool, error) {
+	path := filepath.Join(rootDir, changelogPath)
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		return "", false, fmt.Errorf("read changelog: %w", err)
+	}
+
+	match := releasedSectionRe.FindStringSubmatch(string(contents))
+	if match == nil {
+		return "", false, nil
+	}
+	return match[1], true, nil
+}
+
 func parseSemver(version string) (semver, error) {
 	parts := strings.Split(strings.TrimSpace(version), ".")
 	if len(parts) != 3 {