feat(agent): add Zed agent setup

[SaladDay]

Adds mmx agent setup --agent zed.

• Writes Zed settings.json (Linux $XDG_CONFIG_HOME/~/.config/zed, macOS ~/.config/zed, Windows %APPDATA%\Zed), merging JSONC and keeping existing settings: language_models.openai_compatible.minimax (region endpoint, MiniMax-M3.1-Flash + MiniMax-M3, Flash 512K by default / 1M via --m31-context-window, reasoning_effort never none) and agent.default_model.
• API key is not written: Zed reads it from the OS keychain or MINIMAX_API_KEY; setup prints a one-line hint.
• Zed is configuration-only (no installer).

Tested: unit tests + lint pass; agent setup --agent zed --region cn with a Token Plan key against the CN API; Zed 1.24.2 GUI: basic chat, tool calls (search + read file), effort Minimal and Max all work.

Note: Zed only sends reasoning_effort when the thread's thinking toggle is on, and MiniMax returns reasoning_content only when reasoning_effort is sent, so thinking blocks may not show in Zed. No config field changes this.

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