feat(close): Add --suggest-next flag to show newly unblocked issues (GH#679)

When closing an issue, the new --suggest-next flag returns a list of
issues that became unblocked (ready to work on) as a result of the close.

This helps agents and users quickly identify what work is now available
after completing a blocker.

Example:
  $ bd close bd-5 --suggest-next
  ✓ Closed bd-5: Completed

  Newly unblocked:
    • bd-7 "Implement feature X" (P1)
    • bd-8 "Write tests for X" (P2)

Implementation:
- Added GetNewlyUnblockedByClose to storage interface
- Implemented efficient single-query for SQLite using blocked_issues_cache
- Added SuggestNext field to CloseArgs in RPC protocol
- Added CloseResult type for structured response
- CLI handles both daemon and direct modes

Thanks to @kraitsura for the detailed feature request and design.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/storage_test.go

@@ -98,6 +98,9 @@ func (m *mockStorage) GetEpicsEligibleForClosure(ctx context.Context) ([]*types.
 func (m *mockStorage) GetStaleIssues(ctx context.Context, filter types.StaleFilter) ([]*types.Issue, error) {
 	return nil, nil
 }
+func (m *mockStorage) GetNewlyUnblockedByClose(ctx context.Context, closedIssueID string) ([]*types.Issue, error) {
+	return nil, nil
+}
 func (m *mockStorage) AddComment(ctx context.Context, issueID, actor, comment string) error {
 	return nil
 }