feat(releaser): add --verbose flag for configuration and decision tracing

- Prints a configuration table on startup showing each key, its value,
  and the source (default / config file / env: VARNAME / flag: --name)
- Lists every commit since the last tag with its parsed type and
  the version-bump decision (feat/fix/breaking → patch bump, or ignored)
- Explains the final version choice: highest commit type → next tag
- All verbose output goes to stderr so it never pollutes stdout captures
- Sources tracking wired through config.LoadWithSources and
  ApplyEnvWithSources; LoadWithSources uses a two-pass approach to
  detect which YAML fields were explicitly set vs defaulted

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -50,42 +50,123 @@ func defaults() Config {
 	}
 }
 
+// Sources records where each config value came from.
+// Keys are "section.field" (e.g. "git.tag_prefix").
+// Values are one of: "default", "config file", "env: VARNAME", "flag: --flag-name".
+type Sources map[string]string
+
+func defaultSources() Sources {
+	return Sources{
+		"git.tag_prefix":      "default",
+		"git.branch_pattern":  "default",
+		"git.commit_message":  "default",
+		"git.author_name":     "default",
+		"git.author_email":    "default",
+		"maven.pom_path":      "default",
+		"gitlab.url":          "default",
+		"gitlab.token":        "default",
+		"gitlab.project":      "default",
+	}
+}
+
 // Load reads .releaser.yml from dir and merges it over the defaults.
 // Missing file is not an error — defaults are returned as-is.
 func Load(dir string) (Config, error) {
+	cfg, _, err := LoadWithSources(dir)
+	return cfg, err
+}
+
+// LoadWithSources is like Load but also returns a Sources map recording where each
+// value came from ("default" or "config file").
+func LoadWithSources(dir string) (Config, Sources, error) {
 	cfg := defaults()
+	src := defaultSources()
 
 	data, err := os.ReadFile(filepath.Join(dir, filename))
 	if errors.Is(err, os.ErrNotExist) {
-		return cfg, nil
+		return cfg, src, nil
 	}
 	if err != nil {
-		return cfg, fmt.Errorf("read %s: %w", filename, err)
+		return cfg, src, fmt.Errorf("read %s: %w", filename, err)
 	}
 
+	// Unmarshal into cfg (merges over defaults).
 	if err := yaml.Unmarshal(data, &cfg); err != nil {
-		return cfg, fmt.Errorf("parse %s: %w", filename, err)
+		return cfg, src, fmt.Errorf("parse %s: %w", filename, err)
 	}
 
-	return cfg, nil
+	// Detect which fields the file explicitly set by unmarshaling into a zero overlay.
+	var overlay Config
+	_ = yaml.Unmarshal(data, &overlay)
+	if overlay.Git.TagPrefix != "" {
+		src["git.tag_prefix"] = "config file"
+	}
+	if overlay.Git.BranchPattern != "" {
+		src["git.branch_pattern"] = "config file"
+	}
+	if overlay.Git.CommitMessage != "" {
+		src["git.commit_message"] = "config file"
+	}
+	if overlay.Git.AuthorName != "" {
+		src["git.author_name"] = "config file"
+	}
+	if overlay.Git.AuthorEmail != "" {
+		src["git.author_email"] = "config file"
+	}
+	if overlay.Maven.PomPath != "" {
+		src["maven.pom_path"] = "config file"
+	}
+	if overlay.GitLab.URL != "" {
+		src["gitlab.url"] = "config file"
+	}
+	if overlay.GitLab.Token != "" {
+		src["gitlab.token"] = "config file"
+	}
+	if overlay.GitLab.Project != "" {
+		src["gitlab.project"] = "config file"
+	}
+
+	return cfg, src, nil
 }
 
 // ApplyEnv fills empty GitLab fields from the standard GitLab CI environment variables.
 // Values already set in the config file are never overwritten.
 func (c *Config) ApplyEnv() {
+	c.ApplyEnvWithSources(nil)
+}
+
+// ApplyEnvWithSources is like ApplyEnv but records the env var name in src for each
+// field it fills. src may be nil.
+func (c *Config) ApplyEnvWithSources(src Sources) {
 	if c.GitLab.Token == "" {
-		c.GitLab.Token = os.Getenv("GITLAB_TOKEN")
+		if v := os.Getenv("GITLAB_TOKEN"); v != "" {
+			c.GitLab.Token = v
+			if src != nil {
+				src["gitlab.token"] = "env: GITLAB_TOKEN"
+			}
+		}
 	}
 	if c.GitLab.URL == "" {
 		// CI_SERVER_URL is the cleanest source ("https://gitlab.example.com")
-		c.GitLab.URL = os.Getenv("CI_SERVER_URL")
+		if v := os.Getenv("CI_SERVER_URL"); v != "" {
+			c.GitLab.URL = v
+			if src != nil {
+				src["gitlab.url"] = "env: CI_SERVER_URL"
+			}
+		}
 	}
 	if c.GitLab.Project == "" {
 		// Prefer numeric ID; fall back to namespace/project path
 		if id := os.Getenv("CI_PROJECT_ID"); id != "" {
 			c.GitLab.Project = id
-		} else {
-			c.GitLab.Project = os.Getenv("CI_PROJECT_PATH")
+			if src != nil {
+				src["gitlab.project"] = "env: CI_PROJECT_ID"
+			}
+		} else if p := os.Getenv("CI_PROJECT_PATH"); p != "" {
+			c.GitLab.Project = p
+			if src != nil {
+				src["gitlab.project"] = "env: CI_PROJECT_PATH"
+			}
 		}
 	}
 }