Summary

• Reuse the current main session's selected model during config-time agent rebuilds when config.model is missing.
• Keep explicit config.model precedence unchanged, so direct UI-provided model selections still win when present.
• Add regressions proving primary-agent config loading falls back to persisted session model instead of snapping back to the default model.

Verification

• bun test src/plugin-handlers/agent-config-handler.test.ts
• bun test src/plugin-handlers/config-handler.test.ts src/plugin/chat-message.test.ts
• bun run typecheck
• bun run build

Context

• This addresses the same general class as fix(agents): preserve uiSelectedModel when agent override has no model (fixes #2351) #2788, but a different gap: later config-time agent rebuilds were ignoring session-scoped model state when config.model was absent.
• Fixes [Bug]: Every time I send a message, the model always resets to "Claude Opus 4.6" in windows desktop #2353

Summary by cubic

Preserves the user's selected model across messages by reusing the main session model during agent rebuilds when config.model is missing, fixing #2353. Explicit config.model still takes precedence.

• Bug Fixes
  • Added resolveCurrentModel to prefer config.model, else fall back to the main session model from getMainSessionID and getSessionModel.
  • Updated applyAgentConfig to use the resolved model so sessions don’t snap back to the default after each send.
  • Added tests to cover session model reuse and precedence over config.model.

^{Written for commit bad70f5. Summary will update on new commits.}