refactor(tui): default create form state

[elezar]

Summary

Add Default implementations for the TUI create-form state so modal openers only set dynamic data. This keeps future form fields centralized in the form defaults instead of repeated constructor literals.

Related Issue

N/A

Changes

• Derived defaults for create-form focus and phase enums.
• Derived defaults for CreateSandboxForm and CreateProviderForm.
• Simplified create-form openers with struct update syntax.

Testing

• /home/elezar/.cargo/bin/cargo fmt
• /home/elezar/.cargo/bin/cargo test -p openshell-tui --lib passes
• mise run pre-commit attempted, blocked in workspace Rust tasks by local z3-sys/z3.h header availability
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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[elezar]

/ok-to-test d872bc7