fix: restore swarm preference and add default_swarm_mode

[mbuckaway]

Related Issue

• MoonshotAI/kimi-code Feature: config option to enable swarm mode by default for new sessions (config.toml default_swarm_mode) MoonshotAI/kimi-code#3433 — per-session swarm mode / default_swarm_mode request.
• MoonshotAI/kimi-code fix: tone down over-proactiveness in the default system prompt MoonshotAI/kimi-code#3028 — "tone down over-proactiveness" system-prompt change (merged).

Problem

After the 0.39.1 upstream sync, the fork prefers spawning sequential Agent subagent tasks over the parallel AgentSwarm tool even when the swarm-mode toggle is on, and the task path appears to ignore coding standards / AGENTS.md rules. The sync's MoonshotAI#3028 removed the "default to taking action / make real changes / deliver the complete change" directives from both engines' system prompts, and MoonshotAI#3433 documents that swarm/goal toggles are now per-session (the button does not persist).

What changed

• Restore the fix: tone down over-proactiveness in the default system prompt MoonshotAI/kimi-code#3028-removed proactiveness/action directives in both system.md files.
• Add guidance to prefer AgentSwarm over a single sequential Agent for decomposable work, and to carry coding-standards/AGENTS.md reading into every subagent prompt.
• Add a default_swarm_mode config option (v1 + v2 engines + protocol) so new sessions can start in Swarm mode, mirroring default_plan_mode.
• Add defaultSwarmMode bootstrap tests; regenerate the config manifest; document the option (en/zh); add a changeset and a fork patch row.

Checklist

• I have read the CONTRIBUTING document.
• I have linked related issues (upstream fix: tone down over-proactiveness in the default system prompt MoonshotAI/kimi-code#3028 / Feature: config option to enable swarm mode by default for new sessions (config.toml default_swarm_mode) MoonshotAI/kimi-code#3433).
• I have added tests that prove the feature works.
• Added a changeset per the gen-changesets rules.
• Updated docs (config-files.md en/zh + regenerated config-manifest.toml).