Add configurable export error handling policies (bd-exug)

Implements flexible error handling for export operations with four policies:
- strict: Fail-fast on any error (default for user exports)
- best-effort: Skip errors with warnings (default for auto-exports)
- partial: Retry then skip with manifest tracking
- required-core: Fail on core data, skip enrichments

Key features:
- Per-project configuration via `bd config set export.error_policy`
- Separate policy for auto-exports: `auto_export.error_policy`
- Retry with exponential backoff (configurable attempts/delay)
- Optional export manifests documenting completeness
- Per-issue encoding error handling

This allows users to choose the right trade-off between data integrity
and system availability for their specific project needs.

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

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

internal/export/executor.go

@@ -0,0 +1,96 @@
+package export
+
+import (
+	"context"
+	"fmt"
+	"os"
+)
+
+// DataType represents a type of data being fetched
+type DataType string
+
+const (
+	DataTypeCore     DataType = "core"       // Issues and dependencies
+	DataTypeLabels   DataType = "labels"     // Issue labels
+	DataTypeComments DataType = "comments"   // Issue comments
+)
+
+// FetchResult holds the result of a data fetch operation
+type FetchResult struct {
+	Success  bool
+	Err      error
+	Warnings []string
+}
+
+// FetchWithPolicy executes a fetch operation with the configured error policy
+func FetchWithPolicy(ctx context.Context, cfg *Config, dataType DataType, desc string, fn func() error) FetchResult {
+	var result FetchResult
+
+	// Determine if this is core data
+	isCore := dataType == DataTypeCore
+
+	// Execute based on policy
+	switch cfg.Policy {
+	case PolicyStrict:
+		// Fail-fast on any error
+		err := RetryWithBackoff(ctx, cfg.RetryAttempts, cfg.RetryBackoffMS, desc, fn)
+		if err != nil {
+			result.Err = err
+			return result
+		}
+		result.Success = true
+
+	case PolicyBestEffort:
+		// Skip errors with warnings
+		err := RetryWithBackoff(ctx, cfg.RetryAttempts, cfg.RetryBackoffMS, desc, fn)
+		if err != nil {
+			warning := fmt.Sprintf("Warning: %s failed, skipping: %v", desc, err)
+			fmt.Fprintf(os.Stderr, "%s\n", warning)
+			result.Warnings = append(result.Warnings, warning)
+			result.Success = false // Data is missing
+			return result
+		}
+		result.Success = true
+
+	case PolicyPartial:
+		// Retry with backoff, then skip with manifest entry
+		err := RetryWithBackoff(ctx, cfg.RetryAttempts, cfg.RetryBackoffMS, desc, fn)
+		if err != nil {
+			warning := fmt.Sprintf("Warning: %s failed after retries, skipping: %v", desc, err)
+			fmt.Fprintf(os.Stderr, "%s\n", warning)
+			result.Warnings = append(result.Warnings, warning)
+			result.Success = false
+			return result
+		}
+		result.Success = true
+
+	case PolicyRequiredCore:
+		// Fail on core data, skip enrichments
+		if isCore {
+			err := RetryWithBackoff(ctx, cfg.RetryAttempts, cfg.RetryBackoffMS, desc, fn)
+			if err != nil {
+				result.Err = err
+				return result
+			}
+			result.Success = true
+		} else {
+			// Best-effort for enrichments
+			err := RetryWithBackoff(ctx, cfg.RetryAttempts, cfg.RetryBackoffMS, desc, fn)
+			if err != nil {
+				warning := fmt.Sprintf("Warning: %s (enrichment) failed, skipping: %v", desc, err)
+				fmt.Fprintf(os.Stderr, "%s\n", warning)
+				result.Warnings = append(result.Warnings, warning)
+				result.Success = false
+				return result
+			}
+			result.Success = true
+		}
+
+	default:
+		// Unknown policy, fail-fast as safest option
+		result.Err = fmt.Errorf("unknown error policy: %s", cfg.Policy)
+		return result
+	}
+
+	return result
+}