Add repo-local Codex skills for dependency updates and UX review

[nathanhuh]

Add repo-local Codex skills for routine dependency maintenance and TUI UX review workflows.

Scope:

• Add unic-update-dependencies skill metadata and instructions.
• Add unic-ux-review skill instructions.
• Add an isolated unic TUI capture helper for repeatable UX screenshots.

Validation:

• make test
• make build
• skill YAML/syntax checks