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/contracts/channels.ts

@@ -27,7 +27,7 @@ export const ipcChannels = {
   cancelSessionTurn: 'sessions:cancel-turn',
   resolveSessionApproval: 'sessions:resolve-approval',
   querySessions: 'sessions:query',
-  updateScratchpadSessionConfig: 'sessions:update-scratchpad-config',
+  updateSessionModelConfig: 'sessions:update-model-config',
   selectProject: 'selection:project',
   selectPattern: 'selection:pattern',
   selectSession: 'selection:session',