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/util.go

@@ -3,7 +3,6 @@ package sqlite
 import (
 	"context"
 	"database/sql"
-	"fmt"
 	"strings"
 )
 
@@ -24,7 +23,7 @@ func (s *SQLiteStorage) BeginTx(ctx context.Context) (*sql.Tx, error) {
 func (s *SQLiteStorage) withTx(ctx context.Context, fn func(*sql.Tx) error) error {
 	tx, err := s.db.BeginTx(ctx, nil)
 	if err != nil {
-		return fmt.Errorf("failed to begin transaction: %w", err)
+		return wrapDBError("begin transaction", err)
 	}
 	defer func() { _ = tx.Rollback() }()
 
@@ -33,7 +32,7 @@ func (s *SQLiteStorage) withTx(ctx context.Context, fn func(*sql.Tx) error) erro
 	}
 
 	if err := tx.Commit(); err != nil {
-		return fmt.Errorf("failed to commit transaction: %w", err)
+		return wrapDBError("commit transaction", err)
 	}
 
 	return nil