Outdated skill names in SDD guide

[sentisso]

The guide for SDD (see spec-driven-development.md) contains multiple instances of outdated or incorrect skill names, for example this section:

Create a task file with the initial prompt:

/add-task "Design and implement authentication middleware with JWT support"

The correct skill name would be prefixed with sdd: (see add-task/SKILL.md)

[sentisso]

It's possible that incorrect skill names occur in multiple places in the documentation, but these were the ones I noticed when following the SDD guide.

[LeoVS09]

@sentisso thank you for reporting. We started removing prefixes. It looks like this doc was updated too early. We will fix it soon.