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>

cmd/bd/init.go

@@ -371,6 +371,7 @@ With --no-db: creates .beads/ directory and issues.jsonl file instead of SQLite
 			}
 		}
 		if hasIssues {
+			yellow := color.New(color.FgYellow).SprintFunc()
 			fmt.Printf("%s Setup incomplete. Some issues were detected:\n", yellow("⚠"))
 			// Show just the warnings/errors, not all checks
 			for _, check := range doctorResult.Checks {