Add Community Friends section to README (#1956)

* Add Community Friends section with cc-sdd

Adds a new "Community Friends" section to the README for projects that
extend or build on Spec Kit. Starts with cc-sdd, a Claude Code plugin
that layers composable traits (quality gates, worktree isolation, agent
teams) on top of Spec Kit's core workflow.

Suggested by @mnriem in discussion #1889.

Assisted-By: 🤖 Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Update cc-sdd repo URL after rename

Assisted-By: 🤖 Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Mention Superpowers explicitly in cc-sdd description

Assisted-By: 🤖 Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>