feat: add oh-my-pi setup and default text to M3.1 Flash Preview

[jellyjelly814]

Summary

text chat, text repl, and the SDK still fall back to MiniMax-M3, while agent setup already prefers M3.1 for subscription keys. Use a shared MiniMax-M3.1-Flash-Preview default and add Oh my Pi as the seventh supported agent.

• Keep explicit model choices and saved CLI defaults authoritative. Preserve agent setup's MiniMax-M3 fallback for pay-as-you-go keys, and document the preview's M Plan / MiniMax Code availability.
• Add --agent oh-my-pi (alias omp), executable detection, --all selection, and interactive installation through the official native binary installers.
• Write Oh my Pi's native models.yml / config.yml for China and global endpoints. Honor profile/environment paths and existing .yaml files, preserve unrelated settings, and use the existing backup/private-write safeguards. Ask users with legacy JSON-only configuration to migrate with omp before setup.
• Configure M3.1 adaptive thinking with all five supported effort levels and max as the default. Update English/Chinese documentation and regression coverage.
• Give the installer timeout test 2 seconds for cold shell startup; it still verifies the timeout error and termination of the actual descendant process.

Validation

• bun test: 599 passed, 0 failed.
• bun run typecheck and bun run build: passed.
• bun run lint: no errors; one existing warning in test/sdk/speech.test.ts.
• Built CLI smoke checks cover model selection and Oh my Pi setup.
• Oh my Pi 18.10.1 loaded generated China/global configurations. A local Anthropic mock confirmed the request uses MiniMax-M3.1-Flash-Preview, adaptive thinking, and effort: max.

References: MiniMax China model guide, global model guide, Oh my Pi.

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