Feature: config option to enable swarm mode by default for new sessions (config.toml default_swarm_mode)

Feature request

Add a config option (e.g. default_swarm_mode = true in config.toml, similar to default_plan_mode / default_permission_mode) so that new sessions start with swarm mode enabled by default.

Motivation

Users who work primarily in kimi web (and the TUI) may want swarm mode on for every session without manually toggling it each time. Today swarm mode is a per-session runtime toggle:

• TUI: /swarm on|off (or /swarm <task>, which auto-disables when the turn completes)
• Web UI: the swarm toggle in the composer + menu

The 0.39 changelog notes that swarm/goal toggles are now kept per session in the web UI ("each session now keeps its own toggles"), which means the preference cannot persist across sessions. There is also no documented config.toml key or environment variable to make swarm mode default-on — the existing [swarm] section only controls timeout_ms for AgentSwarm subagents.

Proposed behavior

• default_swarm_mode = true (top-level, default false) makes every new session start with swarm mode on, matching how default_plan_mode works for Plan mode.
• Resumed sessions keep the toggle state recorded for that session (consistent with current per-session toggle semantics).
• The per-session toggle (/swarm on|off / web UI toggle) continues to override the default within the session.
• When swarm mode starts on via config and permission mode is manual, the same auto/yolo permission prompt that /swarm triggers today should apply.

Alternatives considered

• A KIMI_CODE_SWARM_DEFAULT=1 environment variable, alongside the config key, for containers/CI.

Start with config.toml and trace how default_plan_mode is read and applied when a new session starts. Compare that flow with the /swarm toggle and the web UI composer toggle. Done means a top-level default_swarm_mode setting enables swarm mode only for new sessions, while resumed sessions and per-session overrides retain their current behavior.