[codex] Fetch dynamic Pi models and add arrow picker

[Dankosik]

Summary

• fetch GonkaGate model IDs from /v1/models instead of using a checked-in curated registry
• build Pi provider config, --model validation, JSON output model IDs, and defaults from the fetched model list
• replace the numbered model prompt with an arrow-key TTY picker while preserving non-TTY defaults
• update README, PRD, AGENTS, mirrored skills, changelog, and contract tests for the dynamic model contract

Validation

• npm run ci