feat: upgrade MiniMax default model to M2.7

[octo-patch]

Summary

• Add MiniMax-M2.7 and MiniMax-M2.7-highspeed to the model selection list
• Set MiniMax-M2.7 as the new default model for all free-mode agents (base2-free, editor-lite, code-reviewer-lite, librarian, tmux-cli)
• Update FREE_MODE_AGENT_MODELS allowlist to use M2.7
• Retain all previous models (M2.5) as available alternatives
• Update all documentation references (what-models, modes, FAQ, how-does-it-work, freebuff README/SPEC)
• Update related unit tests

Why

MiniMax-M2.7 is the latest flagship model with enhanced reasoning and coding capabilities.

Changes (17 files)

• Agent configs: base2.ts, editor.ts, librarian.ts, code-reviewer-lite.ts, tmux-cli.ts
• Type definitions: agent-definition.ts (3 copies: agents/, .agents/, common/templates/)
• Free mode config: free-agents.ts
• Tests: editor.test.ts
• Docs: what-models.mdx, modes.mdx, faq.mdx, how-does-it-work.mdx, freebuff/README.md, freebuff/SPEC.md, freebuff home-client.tsx

Testing

• Unit tests updated and passing (33/33 pass)
• All model references verified: only default/agent model references updated; vendor-specific infrastructure model IDs (Fireworks, SiliconFlow, CanopyWave) intentionally kept unchanged