fix: handle bd --no-daemon exit code 0 bug on not-found (#339)

When bd --no-daemon show <id> does not find an issue, it incorrectly exits
with code 0 (success) but writes the error to stderr and leaves stdout empty.
This causes JSON parse failures throughout gt when code tries to unmarshal
the empty stdout.

This PR handles the bug defensively in all affected code paths:
- beads.go run(): Detect empty stdout + non-empty stderr as error
- beads.go wrapError(): Add 'no issue found' to ErrNotFound patterns
- sling.go: Check len(out) == 0 in multiple functions
- convoy.go getIssueDetails(): Check stdout.Len() == 0
- prime_molecule.go: Check stdout.Len() == 0

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

internal/cmd/convoy.go

@@ -1186,6 +1186,10 @@ func getIssueDetails(issueID string) *issueDetails {
 	if err := showCmd.Run(); err != nil {
 		return nil
 	}
+	// Handle bd --no-daemon exit 0 bug: empty stdout means not found
+	if stdout.Len() == 0 {
+		return nil
+	}
 
 	var issues []struct {
 		ID        string `json:"id"`