Kanata: Add sequences (defseq) UI support

[malpern]

Problem

Kanata supports sequences (defseq) for git shortcuts and Vim leader sequences, but KeyPath only parses them - users can't create/edit sequences via UI.

Impact

• Can't create git shortcuts (e.g., "gs" → "git status")
• Can't create Vim leader sequences
• Power users must manually edit config

Fix

1. Model defseq in Swift config types
2. Add sequence editor UI
3. Generate defseq blocks in config

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 4
• Kanata docs: defseq syntax