Add epic closure management commands (fixes #62)

- Add 'bd epic status' to show epic completion with child progress
- Add 'bd epic close-eligible' to bulk-close completed epics
- Add GetEpicsEligibleForClosure() storage method
- Update 'bd stats' to show count of epics ready to close
- Add EpicStatus type for tracking epic/child relationships
- Support --eligible-only, --dry-run, and --json flags
- Fix golangci-lint config version requirement

Addresses GitHub issue #62 - epics now have visibility and
management tools for closure when all children are complete.

Amp-Thread-ID: https://ampcode.com/threads/T-e8ac3f48-f0cf-4858-8e8f-aace2481c30d
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -36,6 +36,7 @@ type Storage interface {
 	// Ready Work & Blocking
 	GetReadyWork(ctx context.Context, filter types.WorkFilter) ([]*types.Issue, error)
 	GetBlockedIssues(ctx context.Context) ([]*types.BlockedIssue, error)
+	GetEpicsEligibleForClosure(ctx context.Context) ([]*types.EpicStatus, error)
 
 	// Events
 	AddComment(ctx context.Context, issueID, actor, comment string) error