Add command-based AI model routing

[itsflaid]

Summary

• route devmap ask to llama-3.1-8b-instant
• keep standard analyze on openai/gpt-oss-20b
• route analyze --deep to openai/gpt-oss-120b
• use openai/gpt-oss-20b as the model-unavailable fallback
• add devmap config model for global overrides
• support devmap config model auto to restore command defaults
• preserve existing provider credentials when changing models
• update PRD, command reference, architecture, and maintainer testing docs

Verification

• pnpm test:cli (57/57)
• pnpm build:cli
• pnpm test:package-e2e
• git diff --check

Closes #15