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/manifest.go

@@ -0,0 +1,65 @@
+package export
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// WriteManifest writes an export manifest alongside the JSONL file
+func WriteManifest(jsonlPath string, manifest *Manifest) error {
+	// Derive manifest path from JSONL path
+	manifestPath := strings.TrimSuffix(jsonlPath, ".jsonl") + ".manifest.json"
+
+	// Marshal manifest
+	data, err := json.MarshalIndent(manifest, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal manifest: %w", err)
+	}
+
+	// Create temp file for atomic write
+	dir := filepath.Dir(manifestPath)
+	base := filepath.Base(manifestPath)
+	tempFile, err := os.CreateTemp(dir, base+".tmp.*")
+	if err != nil {
+		return fmt.Errorf("failed to create temp manifest file: %w", err)
+	}
+	tempPath := tempFile.Name()
+	defer func() {
+		_ = tempFile.Close()
+		_ = os.Remove(tempPath)
+	}()
+
+	// Write manifest
+	if _, err := tempFile.Write(data); err != nil {
+		return fmt.Errorf("failed to write manifest: %w", err)
+	}
+
+	// Close before rename
+	_ = tempFile.Close()
+
+	// Atomic replace
+	if err := os.Rename(tempPath, manifestPath); err != nil {
+		return fmt.Errorf("failed to replace manifest file: %w", err)
+	}
+
+	// Set appropriate file permissions (0600: rw-------)
+	if err := os.Chmod(manifestPath, 0600); err != nil {
+		// Non-fatal, just log
+		fmt.Fprintf(os.Stderr, "Warning: failed to set manifest permissions: %v\n", err)
+	}
+
+	return nil
+}
+
+// NewManifest creates a new export manifest
+func NewManifest(policy ErrorPolicy) *Manifest {
+	return &Manifest{
+		ExportedAt:  time.Now(),
+		ErrorPolicy: string(policy),
+		Complete:    true, // Will be set to false if any data is missing
+	}
+}