feat(agent): keep only M3.1 and M3 in agent setup models

[SaladDay]

We're the MiniMax team. This keeps mmx agent setup focused on the current models: only MiniMax-M3.1-Flash-Preview and MiniMax-M3 are offered.

Changes

• Remove MiniMax-M2.7 and MiniMax-M2.7-highspeed from the agent setup model list (src/agent/types.ts). All agents (Claude Code, Codex, Grok, OpenCode, Hermes, Pi, Oh my Pi) and the --model choices derive from this list, so the Claude Code model picker, Codex catalog, and per-agent model entries now list only M3.1 and M3.
• Drop the now-unreachable M2-only branches (-highspeed description, 204.8K context text, Hermes context-window fallback).
• Other commands (text chat, speech, etc.) are untouched.

Tests

• Updated agent configurator and setup tests; bun run typecheck, bun run lint, and bun test pass.
• Ran the built CLI with mmx agent setup for Claude Code, Codex, Grok, OpenCode, Hermes, and Pi against the CN endpoint: written configs contain only M3.1 and M3, and claude -p returns a normal response on M3.1.

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