feat(completion): add dynamic shell completions for issue IDs (#935)

Add intelligent shell completions that query the database to provide
issue ID suggestions with titles for commands that take IDs as arguments.

Changes:
- Add issueIDCompletion function that queries storage for all issues
- Register completion for show, update, close, edit, defer, undefer, reopen
- Add comprehensive test suite with 3 test cases
- Completions display ID with title as description (ID\tTitle format)

The completion function opens the database (read-only) and filters issues
based on the partially typed prefix, providing a better UX for commands
that require issue IDs.

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

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

cmd/bd/reopen.go

@@ -138,5 +138,6 @@ This is more explicit than 'bd update --status open' and emits a Reopened event.
 }
 func init() {
 	reopenCmd.Flags().StringP("reason", "r", "", "Reason for reopening")
+	reopenCmd.ValidArgsFunction = issueIDCompletion
 	rootCmd.AddCommand(reopenCmd)
 }