docs: add design spec for Dart and Flutter support

[zackkitzmiller]

Summary

• Adds docs/superpowers/specs/2026-07-12-dart-flutter-support-design.md, the approved design for new initium dart and initium flutter commands
• Follows the existing per-language generator pattern (Ruby/Python/Node/Go/Rust/Bash): new generator files, ProjectType auto-detection (via pubspec.yaml sniffing), CLI wiring, git hooks, and a test plan
• Design was worked through via brainstorming and approved before writing; implementation plan is the next step

Test plan

• Docs-only change, no code affected
• Follow-up PR will implement the plan and add generator/CLI/hook tests per the spec's Testing section

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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