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/preload/index.ts

@@ -34,8 +34,8 @@ const api: ElectronApi = {
   sendSessionMessage: (input) => ipcRenderer.invoke(ipcChannels.sendSessionMessage, input),
   cancelSessionTurn: (input) => ipcRenderer.invoke(ipcChannels.cancelSessionTurn, input),
   resolveSessionApproval: (input) => ipcRenderer.invoke(ipcChannels.resolveSessionApproval, input),
-  updateScratchpadSessionConfig: (input) =>
-    ipcRenderer.invoke(ipcChannels.updateScratchpadSessionConfig, input),
+  updateSessionModelConfig: (input) =>
+    ipcRenderer.invoke(ipcChannels.updateSessionModelConfig, input),
   querySessions: (input) => ipcRenderer.invoke(ipcChannels.querySessions, input),
   selectProject: (projectId) => ipcRenderer.invoke(ipcChannels.selectProject, projectId),
   selectPattern: (patternId) => ipcRenderer.invoke(ipcChannels.selectPattern, patternId),