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)