[feat] Auto-derive opinionated config fields (#82)

[feat] Auto-derive opinionated config fields (#80)

Make worktree_dir, default_source, and copy_files optional with
auto-derivation. rimba init now writes only copy_files to
settings.toml. Existing configs with explicit values continue
working unchanged.

Author: lugassawan

cmd/init.go

@@ -101,7 +101,10 @@ directory is already personal.`,
 				return err
 			}
 
-			cfg := config.DefaultConfig(repoName, defaultBranch)
+			// Write minimal config — only copy_files (everything else auto-derived)
+			cfg := &config.Config{
+				CopyFiles: config.DefaultCopyFiles(),
+			}
 
 			if err := os.MkdirAll(dirPath, 0750); err != nil {
 				return fmt.Errorf("failed to create config directory: %w", err)
@@ -117,8 +120,8 @@ directory is already personal.`,
 				}
 			}
 
-			// Create the worktree directory
-			wtDir := filepath.Join(repoRoot, cfg.WorktreeDir)
+			// Create the worktree directory using convention
+			wtDir := filepath.Join(repoRoot, config.DefaultWorktreeDir(repoName))
 			if err := os.MkdirAll(wtDir, 0750); err != nil {
 				return fmt.Errorf("failed to create worktree directory: %w", err)
 			}

cmd/init_test.go

@@ -54,7 +54,7 @@ func TestInitSuccess(t *testing.T) {
 
 	// Verify worktree dir was created
 	repoName := filepath.Base(repoDir)
-	wtDir := filepath.Join(repoDir, "../"+repoName+"-worktrees")
+	wtDir := filepath.Join(repoDir, config.DefaultWorktreeDir(repoName))
 	if _, err := os.Stat(wtDir); os.IsNotExist(err) {
 		t.Errorf("worktree dir not created at %s", wtDir)
 	}

cmd/mcp.go

@@ -1,6 +1,8 @@
 package cmd
 
 import (
+	"path/filepath"
+
 	"github.com/lugassawan/rimba/internal/config"
 	"github.com/lugassawan/rimba/internal/git"
 	mcppkg "github.com/lugassawan/rimba/internal/mcp"
@@ -33,6 +35,14 @@ To configure in Claude Code, add to .claude/settings.json:
 
 		// Config is optional — some tools work without it.
 		cfg, _ := config.Resolve(repoRoot)
+		if cfg != nil {
+			repoName := filepath.Base(repoRoot)
+			var defaultBranch string
+			if cfg.DefaultSource == "" {
+				defaultBranch, _ = git.DefaultBranch(r)
+			}
+			cfg.FillDefaults(repoName, defaultBranch)
+		}
 
 		hctx := &mcppkg.HandlerContext{
 			Runner:   r,

cmd/root.go

@@ -1,6 +1,7 @@
 package cmd
 
 import (
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -57,6 +58,18 @@ var rootCmd = &cobra.Command{
 		if err != nil {
 			return err
 		}
+
+		// Auto-derive missing fields
+		repoName := filepath.Base(repoRoot)
+		var defaultBranch string
+		if cfg.DefaultSource == "" {
+			defaultBranch, err = git.DefaultBranch(r)
+			if err != nil {
+				return err
+			}
+		}
+		cfg.FillDefaults(repoName, defaultBranch)
+
 		cmd.SetContext(config.WithConfig(cmd.Context(), cfg))
 		return nil
 	},

internal/config/config.go

@@ -2,7 +2,6 @@ package config
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -21,8 +20,8 @@ const (
 )
 
 type Config struct {
-	WorktreeDir   string   `toml:"worktree_dir"`
-	DefaultSource string   `toml:"default_source"`
+	WorktreeDir   string   `toml:"worktree_dir,omitempty"`
+	DefaultSource string   `toml:"default_source,omitempty"`
 	CopyFiles     []string `toml:"copy_files"`
 	PostCreate    []string `toml:"post_create,omitempty"`
 
@@ -44,12 +43,6 @@ type ModuleConfig struct {
 	WorkDir  string `toml:"work_dir,omitempty"`
 }
 
-// Validation error messages for required config fields.
-const (
-	ErrMsgEmptyWorktreeDir   = "worktree_dir must not be empty"
-	ErrMsgEmptyDefaultSource = "default_source must not be empty"
-)
-
 // IsAutoDetectDeps returns whether automatic dependency detection is enabled.
 // Defaults to true when Deps or AutoDetect is not configured.
 func (c *Config) IsAutoDetectDeps() bool {
@@ -59,29 +52,41 @@ func (c *Config) IsAutoDetectDeps() bool {
 	return *c.Deps.AutoDetect
 }
 
-// Validate checks that required config fields are present.
+// Validate checks config fields for consistency.
 func (c *Config) Validate() error {
-	var errs []error
+	return nil
+}
+
+// DefaultWorktreeDir returns the conventional worktree directory path for a repo.
+func DefaultWorktreeDir(repoName string) string {
+	return "../" + repoName + "-worktrees"
+}
+
+// DefaultCopyFiles returns the default list of files copied to new worktrees.
+func DefaultCopyFiles() []string {
+	return []string{".env", ".env.local", ".envrc", ".tool-versions"}
+}
+
+// FillDefaults fills missing config fields with auto-derived values.
+// repoName is used for WorktreeDir; defaultBranch is used for DefaultSource.
+func (c *Config) FillDefaults(repoName, defaultBranch string) {
 	if c.WorktreeDir == "" {
-		errs = append(errs, errors.New(ErrMsgEmptyWorktreeDir))
+		c.WorktreeDir = DefaultWorktreeDir(repoName)
 	}
 	if c.DefaultSource == "" {
-		errs = append(errs, errors.New(ErrMsgEmptyDefaultSource))
+		c.DefaultSource = defaultBranch
 	}
-	if len(errs) > 0 {
-		return fmt.Errorf("invalid config: %w", errors.Join(errs...))
+	if c.CopyFiles == nil {
+		c.CopyFiles = DefaultCopyFiles()
 	}
-	return nil
 }
 
 type ctxKey struct{}
 
 func DefaultConfig(repoName, defaultBranch string) *Config {
-	return &Config{
-		WorktreeDir:   "../" + repoName + "-worktrees",
-		DefaultSource: defaultBranch,
-		CopyFiles:     []string{".env", ".env.local", ".envrc", ".tool-versions"},
-	}
+	cfg := &Config{}
+	cfg.FillDefaults(repoName, defaultBranch)
+	return cfg
 }
 
 func Load(path string) (*Config, error) {