fix(orphans): honor --db flag for cross-repo orphan detection (#1200)

* fix(orphans): honor --db flag for cross-repo orphan detection

Problem:
- `bd orphans --db /path` ignored the --db flag entirely
- FindOrphanedIssues() hardcoded local .beads/ directory

Solution:
- Introduce IssueProvider interface for abstract issue lookup
- Add StorageProvider adapter wrapping Storage instances
- Update FindOrphanedIssues to accept provider instead of path
- Wire orphans command to create provider from --db flag

Closes: steveyegge/beads#1196

* test(orphans): add cross-repo and provider tests for --db flag fix

- Add TestFindOrphanedIssues_WithMockProvider (table-driven, UT-01 through UT-09)
- Add TestFindOrphanedIssues_CrossRepo (validates --db flag honored)
- Add TestFindOrphanedIssues_LocalProvider (backward compat RT-01)
- Add TestFindOrphanedIssues_ProviderError (error handling UT-07)
- Add TestFindOrphanedIssues_IntegrationCrossRepo (IT-02 full)
- Add TestLocalProvider_* unit tests

Coverage for IssueProvider interface and cross-repo orphan detection.

* docs: add bd orphans command to CLI reference

Document the orphan detection command including the cross-repo
workflow enabled by the --db flag fix in this PR.

internal/storage/provider.go

@@ -0,0 +1,59 @@
+// Package storage defines the interface for issue storage backends.
+package storage
+
+import (
+	"context"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// StorageProvider wraps a Storage interface to provide IssueProvider functionality.
+// This adapts the full Storage interface to the minimal IssueProvider interface
+// needed for orphan detection.
+type StorageProvider struct {
+	storage Storage
+	prefix  string // Cached prefix (empty = not cached yet)
+}
+
+// NewStorageProvider creates an IssueProvider backed by a Storage instance.
+func NewStorageProvider(s Storage) *StorageProvider {
+	return &StorageProvider{storage: s}
+}
+
+// GetOpenIssues returns issues that are open or in_progress.
+func (p *StorageProvider) GetOpenIssues(ctx context.Context) ([]*types.Issue, error) {
+	// Use SearchIssues with empty query and status filter
+	// We need to search for both "open" and "in_progress" issues
+	openStatus := types.StatusOpen
+	openIssues, err := p.storage.SearchIssues(ctx, "", types.IssueFilter{Status: &openStatus})
+	if err != nil {
+		return nil, err
+	}
+
+	inProgressStatus := types.StatusInProgress
+	inProgressIssues, err := p.storage.SearchIssues(ctx, "", types.IssueFilter{Status: &inProgressStatus})
+	if err != nil {
+		return nil, err
+	}
+
+	// Combine results
+	return append(openIssues, inProgressIssues...), nil
+}
+
+// GetIssuePrefix returns the configured issue prefix.
+func (p *StorageProvider) GetIssuePrefix() string {
+	// Cache the prefix on first access
+	if p.prefix == "" {
+		ctx := context.Background()
+		prefix, err := p.storage.GetConfig(ctx, "issue_prefix")
+		if err != nil || prefix == "" {
+			p.prefix = "bd" // default
+		} else {
+			p.prefix = prefix
+		}
+	}
+	return p.prefix
+}
+
+// Ensure StorageProvider implements types.IssueProvider
+var _ types.IssueProvider = (*StorageProvider)(nil)