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/ipc.ts

@@ -36,7 +36,7 @@ export interface ResolveSessionApprovalInput {
   decision: ApprovalDecision;
 }
 
-export interface UpdateScratchpadSessionConfigInput {
+export interface UpdateSessionModelConfigInput {
   sessionId: string;
   model: string;
   reasoningEffort?: ReasoningEffort;
@@ -126,7 +126,7 @@ export interface ElectronApi {
   sendSessionMessage(input: SendSessionMessageInput): Promise<void>;
   cancelSessionTurn(input: CancelSessionTurnInput): Promise<void>;
   resolveSessionApproval(input: ResolveSessionApprovalInput): Promise<WorkspaceState>;
-  updateScratchpadSessionConfig(input: UpdateScratchpadSessionConfigInput): Promise<WorkspaceState>;
+  updateSessionModelConfig(input: UpdateSessionModelConfigInput): Promise<WorkspaceState>;
   querySessions(input: QuerySessionsInput): Promise<SessionQueryResult[]>;
   selectProject(projectId?: string): Promise<WorkspaceState>;
   selectPattern(patternId?: string): Promise<WorkspaceState>;