QOL: Add default agent mode preference and per-chat mode isolation

[jjjrmy]

Summary

• Add "Default Mode" setting in Preferences to choose between Plan and Agent mode for new chats
• Fix mode bleeding across chats - mode now persists per-subChat instead of globally
• Refactor from isPlanMode: boolean to AgentMode type for future extensibility

Changes

New Feature: Default Mode Preference

• Added dropdown in Settings → Preferences to set default mode ("Agent" or "Plan")
• New chats now respect the user's default mode preference
• Works for both new workspaces and new sub-chats within existing workspaces

Per-Chat Mode Isolation

• Mode is now tracked per-subChat using subChatModeAtomFamily
• Switching mode in one chat no longer affects other chats
• Mode persists to localStorage and database

Code Quality Improvements

• Replaced isPlanMode: boolean with AgentMode type ("agent" | "plan")
• Added AGENT_MODES array and getNextMode() utility for clean mode cycling
• Removed deprecated isPlanModeAtom global atom
• Removed backwards compatibility wrappers (isPlanMode, setIsPlanMode)
• Fixed duplicate store file (lib/stores/sub-chat-store.ts)

Props Updated (boolean → AgentMode)

Component	Change
AgentsSlashCommand	isPlanMode?: boolean → mode?: AgentMode
AgentSendButton	isPlanMode?: boolean → mode?: AgentMode
DetailsSidebar	isPlanMode: boolean → mode: AgentMode
PlanWidget	isPlanMode?: boolean → mode?: AgentMode
AgentPlanSidebar	Added mode?: AgentMode prop

Future Extensibility

Adding a new mode (e.g., "debug") now only requires:

export type AgentMode = "agent" | "plan" | "debug"
export const AGENT_MODES: AgentMode[] = ["agent", "plan", "debug"]

Test Plan

• Create new workspace → should use default mode from Settings
• Create new sub-chat within existing workspace → should use default mode
• Change default mode in Settings → new chats should use new default
• Toggle mode in existing chat → should NOT affect other chats
• Refresh app → mode should persist per-subChat
• Sidebar icons should update immediately when mode changes
• /plan and /agent commands work correctly
• Shift+Tab mode toggle cycles through modes