Centralize error handling patterns in storage layer (bd-bwk2)

Created internal/storage/sqlite/errors.go with:
- Sentinel errors: ErrNotFound, ErrInvalidID, ErrConflict, ErrCycle
- wrapDBError helpers that auto-convert sql.ErrNoRows to ErrNotFound
- Type-safe error checking with errors.Is() compatibility

Updated error handling across storage layer:
- dirty.go: Added context to error returns, converted sql.ErrNoRows checks
- util.go: Updated withTx to use wrapDBError
- batch_ops.go: Added context wrapping to batch operations
- dependencies.go: Wrapped errors from markIssuesDirtyTx calls
- ids.go: Added error wrapping for ID validation

Also restored sqlite.go that was accidentally deleted in previous commit.

All tests pass. Provides consistent error wrapping with operation context
for better debugging.

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

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

internal/storage/sqlite/errors.go

@@ -0,0 +1,62 @@
+package sqlite
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+// Sentinel errors for common database conditions
+var (
+	// ErrNotFound indicates the requested resource was not found in the database
+	ErrNotFound = errors.New("not found")
+
+	// ErrInvalidID indicates an ID format or validation error
+	ErrInvalidID = errors.New("invalid ID")
+
+	// ErrConflict indicates a unique constraint violation or conflicting state
+	ErrConflict = errors.New("conflict")
+
+	// ErrCycle indicates a dependency cycle would be created
+	ErrCycle = errors.New("dependency cycle detected")
+)
+
+// wrapDBError wraps a database error with operation context
+// It converts sql.ErrNoRows to ErrNotFound for consistent error handling
+func wrapDBError(op string, err error) error {
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, sql.ErrNoRows) {
+		return fmt.Errorf("%s: %w", op, ErrNotFound)
+	}
+	return fmt.Errorf("%s: %w", op, err)
+}
+
+// wrapDBErrorf wraps a database error with formatted operation context
+// It converts sql.ErrNoRows to ErrNotFound for consistent error handling
+func wrapDBErrorf(err error, format string, args ...interface{}) error {
+	if err == nil {
+		return nil
+	}
+	op := fmt.Sprintf(format, args...)
+	if errors.Is(err, sql.ErrNoRows) {
+		return fmt.Errorf("%s: %w", op, ErrNotFound)
+	}
+	return fmt.Errorf("%s: %w", op, err)
+}
+
+// IsNotFound checks if an error is or wraps ErrNotFound
+func IsNotFound(err error) bool {
+	return errors.Is(err, ErrNotFound)
+}
+
+// IsConflict checks if an error is or wraps ErrConflict
+func IsConflict(err error) bool {
+	return errors.Is(err, ErrConflict)
+}
+
+// IsCycle checks if an error is or wraps ErrCycle
+func IsCycle(err error) bool {
+	return errors.Is(err, ErrCycle)
+}