feat: bd ready filters by external dep satisfaction (bd-zmmy)

GetReadyWork now lazily resolves external dependencies at query time:
- External refs (external:project:capability) checked against target DB
- Issues with unsatisfied external deps are filtered from ready list
- Satisfaction = closed issue with provides:<capability> label in target

Key changes:
- Remove FK constraint on depends_on_id to allow external refs
- Add migration 025 to drop FK and recreate views
- Filter external deps in GetReadyWork, not in blocked_issues_cache
- Add application-level validation for orphaned local deps
- Comprehensive tests for external dep resolution

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

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

internal/storage/sqlite/external_deps.go

@@ -12,6 +12,7 @@ package sqlite
 import (
 	"context"
 	"database/sql"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -73,14 +74,27 @@ func CheckExternalDep(ctx context.Context, ref string) *ExternalDepStatus {
 
 	dbPath := cfg.DatabasePath(beadsDir)
 
-	// Open the external database (read-only)
-	db, err := sql.Open("sqlite3", dbPath+"?mode=ro")
+	// Verify database file exists
+	if _, err := os.Stat(dbPath); err != nil {
+		status.Reason = "database file not found: " + dbPath
+		return status
+	}
+
+	// Open the external database
+	// Use regular mode to ensure we can read from WAL-mode databases
+	db, err := sql.Open("sqlite3", dbPath)
 	if err != nil {
-		status.Reason = "cannot open project database"
+		status.Reason = "cannot open project database: " + err.Error()
 		return status
 	}
 	defer func() { _ = db.Close() }()
 
+	// Verify we can ping the database
+	if err := db.Ping(); err != nil {
+		status.Reason = "cannot connect to project database: " + err.Error()
+		return status
+	}
+
 	// Check for a closed issue with provides:<capability> label
 	providesLabel := "provides:" + status.Capability
 	var count int
@@ -92,7 +106,7 @@ func CheckExternalDep(ctx context.Context, ref string) *ExternalDepStatus {
 	`, providesLabel).Scan(&count)
 
 	if err != nil {
-		status.Reason = "database query failed"
+		status.Reason = "database query failed: " + err.Error()
 		return status
 	}