feat: allow model selection in all single-agent sessions

Generalize the scratchpad-only model/reasoning-effort selector to work
in any session whose pattern has exactly one agent. This enables model
switching above the chat input for project 1-on-1 chats, not just
scratchpads.

- Rename ScratchpadSessionConfig -> SessionModelConfig across domain,
  contracts, IPC, preload, and renderer
- Replace isScratchpadProject guard with agents.length === 1 check in
  EryxAppService.updateSessionModelConfig and ChatPane pills gate
- Apply session model config in buildEffectivePattern whenever present,
  regardless of project type
- Seed sessionModelConfig on session creation for any single-agent
  pattern

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/domain/sessionLibrary.ts

@@ -174,7 +174,7 @@ export function duplicateSessionRecord(
     isPinned: false,
     isArchived: false,
     lastError: undefined,
-    scratchpadConfig: session.scratchpadConfig ? { ...session.scratchpadConfig } : undefined,
+    sessionModelConfig: session.sessionModelConfig ? { ...session.sessionModelConfig } : undefined,
     tooling: session.tooling
       ? {
           enabledMcpServerIds: [...session.tooling.enabledMcpServerIds],