feat: add cross-project dependency support - config and external: prefix (bd-66w1, bd-om4a)

Config (bd-66w1):
- Add external_projects config for mapping project names to paths
- Add GetExternalProjects() and ResolveExternalProjectPath() functions
- Add config documentation and tests

External deps (bd-om4a):
- bd dep add accepts external:project:capability syntax
- External refs stored as-is in dependencies table
- GetBlockedIssues includes external deps in blocked_by list
- blocked_issues_cache includes external dependencies
- Add validation and parsing helpers for external refs

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/config.go

@@ -120,6 +120,10 @@ func Initialize() error {
 	// Maps directory patterns to labels for automatic filtering in monorepos
 	v.SetDefault("directory.labels", map[string]string{})
 
+	// External projects for cross-project dependency resolution (bd-h807)
+	// Maps project names to paths for resolving external: blocked_by references
+	v.SetDefault("external_projects", map[string]string{})
+
 	// Read config file if it was found
 	if configFileSet {
 		if err := v.ReadInConfig(); err != nil {
@@ -263,6 +267,43 @@ func GetMultiRepoConfig() *MultiRepoConfig {
 	}
 }
 
+// GetExternalProjects returns the external_projects configuration.
+// Maps project names to paths for cross-project dependency resolution.
+// Example config.yaml:
+//
+//	external_projects:
+//	  beads: ../beads
+//	  gastown: /absolute/path/to/gastown
+func GetExternalProjects() map[string]string {
+	return GetStringMapString("external_projects")
+}
+
+// ResolveExternalProjectPath resolves a project name to its absolute path.
+// Returns empty string if project not configured or path doesn't exist.
+func ResolveExternalProjectPath(projectName string) string {
+	projects := GetExternalProjects()
+	path, ok := projects[projectName]
+	if !ok {
+		return ""
+	}
+
+	// Expand relative paths from config file location or cwd
+	if !filepath.IsAbs(path) {
+		cwd, err := os.Getwd()
+		if err != nil {
+			return ""
+		}
+		path = filepath.Join(cwd, path)
+	}
+
+	// Verify path exists
+	if _, err := os.Stat(path); err != nil {
+		return ""
+	}
+
+	return path
+}
+
 // GetIdentity resolves the user's identity for messaging.
 // Priority chain:
 //  1. flagValue (if non-empty, from --identity flag)

internal/config/config_test.go

@@ -451,6 +451,146 @@ func TestGetIdentity(t *testing.T) {
 	}
 }
 
+func TestGetExternalProjects(t *testing.T) {
+	err := Initialize()
+	if err != nil {
+		t.Fatalf("Initialize() returned error: %v", err)
+	}
+
+	// Test default (empty map)
+	got := GetExternalProjects()
+	if got == nil {
+		t.Error("GetExternalProjects() returned nil, want empty map")
+	}
+	if len(got) != 0 {
+		t.Errorf("GetExternalProjects() = %v, want empty map", got)
+	}
+
+	// Test with Set
+	Set("external_projects", map[string]string{
+		"beads":   "../beads",
+		"gastown": "/absolute/path/to/gastown",
+	})
+
+	got = GetExternalProjects()
+	if len(got) != 2 {
+		t.Errorf("GetExternalProjects() has %d items, want 2", len(got))
+	}
+	if got["beads"] != "../beads" {
+		t.Errorf("GetExternalProjects()[beads] = %q, want \"../beads\"", got["beads"])
+	}
+	if got["gastown"] != "/absolute/path/to/gastown" {
+		t.Errorf("GetExternalProjects()[gastown] = %q, want \"/absolute/path/to/gastown\"", got["gastown"])
+	}
+}
+
+func TestGetExternalProjectsFromConfig(t *testing.T) {
+	// Create a temporary directory for config file
+	tmpDir := t.TempDir()
+
+	// Create a config file with external_projects
+	configContent := `
+external_projects:
+  beads: ../beads
+  gastown: /path/to/gastown
+  other: ./relative/path
+`
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0750); err != nil {
+		t.Fatalf("failed to create .beads directory: %v", err)
+	}
+
+	configPath := filepath.Join(beadsDir, "config.yaml")
+	if err := os.WriteFile(configPath, []byte(configContent), 0600); err != nil {
+		t.Fatalf("failed to write config file: %v", err)
+	}
+
+	// Change to tmp directory
+	t.Chdir(tmpDir)
+
+	// Initialize viper
+	err := Initialize()
+	if err != nil {
+		t.Fatalf("Initialize() returned error: %v", err)
+	}
+
+	// Test that external_projects is loaded correctly
+	got := GetExternalProjects()
+	if len(got) != 3 {
+		t.Errorf("GetExternalProjects() has %d items, want 3", len(got))
+	}
+	if got["beads"] != "../beads" {
+		t.Errorf("GetExternalProjects()[beads] = %q, want \"../beads\"", got["beads"])
+	}
+	if got["gastown"] != "/path/to/gastown" {
+		t.Errorf("GetExternalProjects()[gastown] = %q, want \"/path/to/gastown\"", got["gastown"])
+	}
+	if got["other"] != "./relative/path" {
+		t.Errorf("GetExternalProjects()[other] = %q, want \"./relative/path\"", got["other"])
+	}
+}
+
+func TestResolveExternalProjectPath(t *testing.T) {
+	// Create a temporary directory structure
+	tmpDir := t.TempDir()
+
+	// Create a project directory to resolve to
+	projectDir := filepath.Join(tmpDir, "beads-project")
+	if err := os.MkdirAll(projectDir, 0750); err != nil {
+		t.Fatalf("failed to create project directory: %v", err)
+	}
+
+	// Create config file
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0750); err != nil {
+		t.Fatalf("failed to create .beads directory: %v", err)
+	}
+
+	configContent := `
+external_projects:
+  beads: beads-project
+  missing: nonexistent-path
+  absolute: ` + projectDir + `
+`
+	configPath := filepath.Join(beadsDir, "config.yaml")
+	if err := os.WriteFile(configPath, []byte(configContent), 0600); err != nil {
+		t.Fatalf("failed to write config file: %v", err)
+	}
+
+	// Change to tmp directory
+	t.Chdir(tmpDir)
+
+	// Initialize viper
+	err := Initialize()
+	if err != nil {
+		t.Fatalf("Initialize() returned error: %v", err)
+	}
+
+	// Test resolving a relative path that exists
+	got := ResolveExternalProjectPath("beads")
+	if got != projectDir {
+		t.Errorf("ResolveExternalProjectPath(beads) = %q, want %q", got, projectDir)
+	}
+
+	// Test resolving a path that doesn't exist
+	got = ResolveExternalProjectPath("missing")
+	if got != "" {
+		t.Errorf("ResolveExternalProjectPath(missing) = %q, want empty string", got)
+	}
+
+	// Test resolving a project that isn't configured
+	got = ResolveExternalProjectPath("unknown")
+	if got != "" {
+		t.Errorf("ResolveExternalProjectPath(unknown) = %q, want empty string", got)
+	}
+
+	// Test resolving an absolute path
+	got = ResolveExternalProjectPath("absolute")
+	if got != projectDir {
+		t.Errorf("ResolveExternalProjectPath(absolute) = %q, want %q", got, projectDir)
+	}
+}
+
 func TestGetIdentityFromConfig(t *testing.T) {
 	// Create a temporary directory for config file
 	tmpDir := t.TempDir()