fix(tui): restore startup validation and branding

[shuv1337]

Summary

• defer configured-agent model validation until the model catalog has loaded
• keep warnings for models absent from an authoritative loaded catalog
• restore the shuvcode executable in TUI and Mini continuation commands
• cover startup-pending, genuine-invalid, and resume-branding cases

Root causes

Agent and model catalogs load in parallel. The TUI treated the model catalog pending state as an empty authoritative catalog, so an agent response arriving first emitted a false warning. Runtime model resolution succeeded after the catalog finished loading.

The TUI epilogue and Mini exit splash also retained upstream hardcoded opencode executable names instead of using the fork name.

Validation

• TUI full suite: 258 pass, 1 skip, 0 fail
• focused TUI tests: 5 pass, 0 fail
• focused Mini splash test: 1 pass, 0 fail
• CLI typecheck
• pre-push monorepo typecheck: 32/32