Add rename-prefix command (bd-420)

- Implement bd rename-prefix command with --dry-run and --json flags
- Add prefix validation (max 8 chars, lowercase, starts with letter)
- Update all issue IDs and text references atomically per issue
- Update dependencies, labels, events, and counters
- Fix counter merge to use MAX() to prevent ID collisions
- Update snapshot tables for FK integrity
- Add comprehensive tests for validation and rename workflow
- Document in README.md and AGENTS.md

Known limitation: Each issue updates in its own transaction.
A failure mid-way could leave mixed state. Acceptable for
intended use case (infrequent operation on small DBs).

Amp-Thread-ID: https://ampcode.com/threads/T-7e77b779-bd88-44f2-9f0b-a9f2ccd54d38
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -57,6 +57,11 @@ type Storage interface {
 	SetMetadata(ctx context.Context, key, value string) error
 	GetMetadata(ctx context.Context, key string) (string, error)
 
+	// Prefix rename operations
+	UpdateIssueID(ctx context.Context, oldID, newID string, issue *types.Issue, actor string) error
+	RenameDependencyPrefix(ctx context.Context, oldPrefix, newPrefix string) error
+	RenameCounterPrefix(ctx context.Context, oldPrefix, newPrefix string) error
+
 	// Lifecycle
 	Close() error
 }