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/session.ts

@@ -16,7 +16,7 @@ export type ChatRole = 'system' | 'user' | 'assistant';
 export type SessionStatus = 'idle' | 'running' | 'error';
 export type SessionTitleSource = 'auto' | 'manual';
 
-export interface ScratchpadSessionConfig {
+export interface SessionModelConfig {
   model: string;
   reasoningEffort?: ReasoningEffort;
 }
@@ -43,7 +43,7 @@ export interface SessionRecord {
   isArchived?: boolean;
   messages: ChatMessageRecord[];
   lastError?: string;
-  scratchpadConfig?: ScratchpadSessionConfig;
+  sessionModelConfig?: SessionModelConfig;
   tooling?: SessionToolingSelection;
   approvalSettings?: SessionApprovalSettings;
   pendingApproval?: PendingApprovalRecord;
@@ -63,9 +63,9 @@ export function resolveSessionTitle(
   return buildSessionTitle(pattern, messages);
 }
 
-export function createScratchpadSessionConfig(
+export function createSessionModelConfig(
   pattern: PatternDefinition,
-): ScratchpadSessionConfig | undefined {
+): SessionModelConfig | undefined {
   const primaryAgent = pattern.agents[0];
   if (!primaryAgent) {
     return undefined;
@@ -89,27 +89,27 @@ export function resolveSessionApprovalSettings(
   return normalizeSessionApprovalSettings(session.approvalSettings);
 }
 
-export function resolveScratchpadSessionConfig(
+export function resolveSessionModelConfig(
   session: SessionRecord,
   pattern: PatternDefinition,
-): ScratchpadSessionConfig | undefined {
-  const defaults = createScratchpadSessionConfig(pattern);
+): SessionModelConfig | undefined {
+  const defaults = createSessionModelConfig(pattern);
   if (!defaults) {
     return undefined;
   }
 
-  const overrideModel = session.scratchpadConfig?.model.trim();
+  const overrideModel = session.sessionModelConfig?.model.trim();
   return {
     model: overrideModel || defaults.model,
-    reasoningEffort: session.scratchpadConfig?.reasoningEffort ?? defaults.reasoningEffort,
+    reasoningEffort: session.sessionModelConfig?.reasoningEffort ?? defaults.reasoningEffort,
   };
 }
 
-export function applyScratchpadSessionConfig(
+export function applySessionModelConfig(
   pattern: PatternDefinition,
   session: SessionRecord,
 ): PatternDefinition {
-  const config = resolveScratchpadSessionConfig(session, pattern);
+  const config = resolveSessionModelConfig(session, pattern);
   const primaryAgent = pattern.agents[0];
   if (!config || !primaryAgent) {
     return pattern;

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],