Make Agent the Baseline Effective Chat Mode

Summary

Make local-agent the optimistic automatic baseline across new-chat surfaces while preserving confirmed availability fallbacks and explicit choices.

User state	Automatic effective mode
Pro	Agent
Non-Pro, quota unresolved	Agent
Non-Pro, quota available, no provider yet	Agent provisionally
Non-Pro, quota available, eligible non-Google provider	Agent
Non-Pro, Google-only	Build
Non-Pro, quota confirmed exhausted	Build
Explicit default set to Build	Build

Existing chats with a stored mode remain unchanged. Explicitly selecting Agent remains honored with Google-only, subject to the existing confirmed-quota-exhaustion fallback.

Implementation Changes

• Update effective-default resolution so only freeAgentQuotaAvailable === false triggers the quota fallback. true and undefined use the optimistic Agent baseline unless the user is Google-only or explicitly defaulted to Build.
• Distinguish “no provider” from “Google-only”: no provider remains provisionally Agent, while Google without another eligible provider resolves to Build.
• Treat selectedChatMode: "build" as current UI state rather than the authoritative automatic default. At startup, synchronize implicit Build to the resolved default, while preserving defaultChatMode: "build", not persisting provisional Agent while provider or quota state is unresolved, and retaining the current-session latch for manual selections.
• Represent automatically created chats with chatMode: null, using the existing nullable schema. Keep initialChatMode as an explicit, latched override.
• Resolve null-mode chats dynamically. A provisional Agent chat can become Build before its first turn when Google-only setup or confirmed quota exhaustion is discovered.
• On the first accepted turn, persist the resolved/requested mode to latch the conversation. Main-process resolution must use the latest quota result: confirmed exhaustion latches Build; a still-unresolved status remains Agent.
• Thread the existing isChatModeExplicit signal through first-prompt creation so manual choices are stored immediately while automatic choices remain implicit.
• Do not migrate historical chat rows. Stored Build, Ask, Plan, and Agent chats remain fixed; normal runtime quota fallback still applies to stored Agent chats.

Interfaces and Compatibility

• No database migration or new IPC endpoint is required.
• Clarify the optional initialChatMode contract: present means explicit and persisted; absent means use the automatic effective default.
• Continue using nullable chat.chatMode as the marker for an implicit, not-yet-latched mode.
• Preserve deprecated "agent" → "build" migration, explicit default settings, free-model compatibility behavior, and manual Agent selection with Google.

Test Plan

• Unit-test the resolver matrix:
  • unresolved quota → Agent;
  • available quota → Agent unless Google-only;
  • confirmed exhaustion → Build;
  • no provider → Agent;
  • Google-only → Build;
  • Google plus eligible non-Google → Agent;
  • Pro → Agent;
  • explicit Build default → Build.
• Verify automatic Google-only mode resolves to Build while an explicit Agent choice remains Agent unless quota is confirmed exhausted.
• Test startup synchronization:
  • eligible implicit Build → Agent;
  • explicit Build remains Build;
  • unresolved provider/quota Agent is computed but not persisted;
  • confirmed exhausted and Google-only resolve to Build.
• Test chat creation and latching:
  • automatic chats store null;
  • explicit modes store their value;
  • an empty implicit Agent chat can change to Build after Google setup or confirmed exhaustion;
  • first accepted turn stores the latest resolved mode;
  • existing stored Build chats are untouched.
• Extend first-prompt tests for non-Google → Agent, Google-only → Build, confirmed exhaustion → Build, unresolved quota → Agent, and manual Agent bypassing automatic provider recomputation.
• Run targeted unit/integration suites, then npm run fmt, npm run lint, and npm run ts.

Assumptions

• “Unresolved quota” includes initial loading and quota-check failure; both remain optimistically Agent.
• Only a confirmed exhausted response triggers quota-based Build fallback.
• “Google-only uses Build” applies to automatic defaults; explicit Agent remains allowed.
• Existing conversation rows are never rewritten by startup synchronization.