feat(update): add --type flag to bd update command (GH#522)

Allow changing an issue type (bug, feature, task, epic, chore) via the
bd update command. The storage layer already supported issue_type in
allowedUpdateFields, this just exposes it through the CLI.

Changes:
- Add --type/-t flag to updateCmd in show.go
- Add IssueType field to UpdateArgs in protocol.go
- Handle issue_type in updatesFromArgs in server_issues_epics.go
- Add validation using ParseIssueType before update

Example usage:
  bd update ab-xyz --type epic

Fixes: #522

.beads/.gitignore

@@ -30,3 +30,5 @@ beads.right.meta.json
 !issues.jsonl
 !metadata.json
 !config.json
+deletions.jsonl
+deletions.jsonl.migrated