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/dependencies.go

@@ -147,7 +147,7 @@ func (s *SQLiteStorage) AddDependency(ctx context.Context, dep *types.Dependency
 		// Mark both issues as dirty for incremental export
 		// (dependencies are exported with each issue, so both need updating)
 		if err := markIssuesDirtyTx(ctx, tx, []string{dep.IssueID, dep.DependsOnID}); err != nil {
-			return err
+			return wrapDBError("mark issues dirty after adding dependency", err)
 		}
 
 		return nil
@@ -184,7 +184,7 @@ func (s *SQLiteStorage) RemoveDependency(ctx context.Context, issueID, dependsOn
 
 		// Mark both issues as dirty for incremental export
 		if err := markIssuesDirtyTx(ctx, tx, []string{issueID, dependsOnID}); err != nil {
-			return err
+			return wrapDBError("mark issues dirty after removing dependency", err)
 		}
 
 		return nil