Implement BEADS_DIR environment variable (bd-e16b)

Add BEADS_DIR as a replacement for BEADS_DB to point to the .beads
directory instead of the database file directly.

Rationale:
- With --no-db mode, there's no .db file to point to
- The .beads directory is the logical unit (contains config.yaml, db
  files, jsonl files)
- More intuitive: point to the beads directory not the database file

Implementation:
- Add BEADS_DIR environment variable support to FindDatabasePath()
- Priority order: BEADS_DIR > BEADS_DB > auto-discovery
- Maintain backward compatibility with BEADS_DB (now deprecated)
- Update --no-db mode to respect BEADS_DIR
- Update MCP integration (config.py, bd_client.py)
- Update documentation to show BEADS_DIR as preferred method

Testing:
- Backward compatibility: BEADS_DB still works
- BEADS_DIR works with regular database mode
- BEADS_DIR works with --no-db mode
- Priority: BEADS_DIR takes precedence over BEADS_DB

Follow-up issues for refactoring:
- bd-efe8: Refactor path canonicalization into helper function
- bd-c362: Extract database search logic into helper function

Closes bd-e16b

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/main.go

@@ -177,7 +177,8 @@ var rootCmd = &cobra.Command{
 				// No database found - error out instead of falling back to ~/.beads
 				fmt.Fprintf(os.Stderr, "Error: no beads database found\n")
 				fmt.Fprintf(os.Stderr, "Hint: run 'bd init' to create a database in the current directory\n")
-				fmt.Fprintf(os.Stderr, "      or set BEADS_DB environment variable to specify a database\n")
+				fmt.Fprintf(os.Stderr, "      or set BEADS_DIR to point to your .beads directory\n")
+				fmt.Fprintf(os.Stderr, "      or set BEADS_DB to point to your database file (deprecated)\n")
 				os.Exit(1)
 			}
 		}
@@ -438,13 +439,29 @@ var rootCmd = &cobra.Command{
 		// Handle --no-db mode: write memory storage back to JSONL
 		if noDb {
 			if store != nil {
-				cwd, err := os.Getwd()
-				if err != nil {
-					fmt.Fprintf(os.Stderr, "Error: failed to get current directory: %v\n", err)
-					os.Exit(1)
+				// Determine beads directory (respect BEADS_DIR)
+				var beadsDir string
+				if envDir := os.Getenv("BEADS_DIR"); envDir != "" {
+					// Canonicalize the path
+					if absDir, err := filepath.Abs(envDir); err == nil {
+						if canonical, err := filepath.EvalSymlinks(absDir); err == nil {
+							beadsDir = canonical
+						} else {
+							beadsDir = absDir
+						}
+					} else {
+						beadsDir = envDir
+					}
+				} else {
+					// Fall back to current directory
+					cwd, err := os.Getwd()
+					if err != nil {
+						fmt.Fprintf(os.Stderr, "Error: failed to get current directory: %v\n", err)
+						os.Exit(1)
+					}
+					beadsDir = filepath.Join(cwd, ".beads")
 				}
 
-				beadsDir := filepath.Join(cwd, ".beads")
 				if memStore, ok := store.(*memory.MemoryStorage); ok {
 					if err := writeIssuesToJSONL(memStore, beadsDir); err != nil {
 						fmt.Fprintf(os.Stderr, "Error: failed to write JSONL: %v\n", err)

cmd/bd/nodb.go

@@ -18,14 +18,31 @@ import (
 // This is called when --no-db flag is set
 func initializeNoDbMode() error {
 	// Find .beads directory
-	cwd, err := os.Getwd()
-	if err != nil {
-		return fmt.Errorf("failed to get current directory: %w", err)
+	var beadsDir string
+
+	// Check BEADS_DIR environment variable first
+	if envDir := os.Getenv("BEADS_DIR"); envDir != "" {
+		// Canonicalize the path
+		if absDir, err := filepath.Abs(envDir); err == nil {
+			if canonical, err := filepath.EvalSymlinks(absDir); err == nil {
+				beadsDir = canonical
+			} else {
+				beadsDir = absDir
+			}
+		} else {
+			beadsDir = envDir
+		}
+	} else {
+		// Fall back to current directory
+		cwd, err := os.Getwd()
+		if err != nil {
+			return fmt.Errorf("failed to get current directory: %w", err)
+		}
+		beadsDir = filepath.Join(cwd, ".beads")
 	}
 
-	beadsDir := filepath.Join(cwd, ".beads")
 	if _, err := os.Stat(beadsDir); os.IsNotExist(err) {
-		return fmt.Errorf("no .beads directory found (hint: run 'bd init' first)")
+		return fmt.Errorf("no .beads directory found (hint: run 'bd init' first or set BEADS_DIR)")
 	}
 
 	jsonlPath := filepath.Join(beadsDir, "issues.jsonl")