fix: align Jira token pre-check with actual token resolution

[anotherGoogleFan]

Summary

• centralize token lookup in api.GetAPIToken
• reuse that helper in the root pre-check so config file api_token is treated as a valid credential source
• remove duplicated token source checks from root.go

Testing

• go test ./internal/cmd/root ./api

Notes

• Full go test ./... still has an existing unrelated failure in pkg/tui (TestGetPager) in this environment.