feat(code-mode): per-session model + effort, and keep output on nav (#632)

Two improvements to the Code section:

- Fix: leaving the Code section and returning no longer drops the open
  session's output. The selected session id is persisted to localStorage
  (mirroring the terminal-height pattern) and restored on remount, so the
  right-hand chat pane re-binds instead of falling back to the empty state.

- Feature: choose the coding agent's model and reasoning effort per session.
  Choices are discovered live from the engine (the same list `/model` shows)
  via a new `codeMode:listModelOptions` IPC, cached per agent — never
  hardcoded, so they track whatever the provider currently offers. Claude
  exposes model + effort as separate axes (with explicit Opus/Sonnet/Haiku
  alias rows surfaced for clarity); Codex folds effort into the model id and
  reports no separate effort. Selections persist on the CodeSession and are
  re-applied to the ACP session each turn (best-effort), editable from both
  the new-session dialog and the session header.

apps/x/packages/shared/src/code-sessions.ts

@@ -53,11 +53,34 @@ export const CodeSession = z.object({
     // Where the agent works: the project path, or the worktree path.
     cwd: z.string(),
     worktree: CodeWorktree.optional(),
+    // The coding agent's own model + reasoning effort (applied to the ACP engine,
+    // not the Rowboat-mode LLM). Values come from CODE_AGENT_MODELS /
+    // CODE_AGENT_EFFORTS; unset (or 'default') leaves the engine's own default.
+    agentModel: z.string().optional(),
+    agentEffort: z.string().optional(),
     createdAt: z.iso.datetime(),
     lastActivityAt: z.iso.datetime().optional(),
 });
 export type CodeSession = z.infer<typeof CodeSession>;
 
+// Model + effort choices for the ACP coding agents are discovered live from the
+// engine (the same list `/model` shows), not hardcoded — so they always reflect
+// whatever the provider currently offers. See the `codeMode:listModelOptions`
+// IPC and CodeModeManager.listModelOptions. 'default' is a synthetic sentinel
+// meaning "don't override the engine default".
+//
+// Claude exposes model and effort as two independent options; Codex folds the
+// reasoning effort into the model id ("gpt-5-codex[high]") and so reports no
+// separate effort list. The UI renders whatever each agent advertises.
+export const CodeAgentOption = z.object({ value: z.string(), label: z.string() });
+export type CodeAgentOption = z.infer<typeof CodeAgentOption>;
+
+export const CodeAgentModelOptions = z.object({
+    models: z.array(CodeAgentOption),
+    efforts: z.array(CodeAgentOption),
+});
+export type CodeAgentModelOptions = z.infer<typeof CodeAgentModelOptions>;
+
 export const GitFileState = z.enum(["modified", "added", "deleted", "untracked", "renamed"]);
 export type GitFileState = z.infer<typeof GitFileState>;
 

apps/x/packages/shared/src/ipc.ts

@@ -21,7 +21,7 @@ import { BillingInfoSchema } from './billing.js';
 import { EmailBlockSchema, GmailThreadSchema } from './blocks.js';
 import { PermissionDecision, ApprovalPolicy, CodingAgent } from './code-mode.js';
 import { NotificationSettingsSchema } from './notification-settings.js';
-import { CodeProject, CodeSession, CodeSessionMode, CodeSessionStatus, GitRepoInfo, GitStatusFile } from './code-sessions.js';
+import { CodeProject, CodeSession, CodeSessionMode, CodeSessionStatus, GitRepoInfo, GitStatusFile, CodeAgentModelOptions } from './code-sessions.js';
 
 // ============================================================================
 // Runtime Validation Schemas (Single Source of Truth)
@@ -601,6 +601,10 @@ const ipcSchemas = {
       // chat, the model is fixed once the session's run exists.
       model: z.string().optional(),
       provider: z.string().optional(),
+      // The coding agent's own model + reasoning effort (ACP engine). Unlike the
+      // Rowboat model these are re-applied each turn, so they stay editable.
+      agentModel: z.string().optional(),
+      agentEffort: z.string().optional(),
     }),
     res: z.object({
       session: CodeSession,
@@ -616,12 +620,18 @@ const ipcSchemas = {
   'codeSession:update': {
     req: z.object({
       sessionId: z.string(),
-      patch: CodeSession.pick({ title: true, mode: true, policy: true, agent: true }).partial(),
+      patch: CodeSession.pick({ title: true, mode: true, policy: true, agent: true, agentModel: true, agentEffort: true }).partial(),
     }),
     res: z.object({
       session: CodeSession,
     }),
   },
+  // Live model + effort choices for a coding agent, discovered from the engine
+  // (cached per agent in the main process). Mirrors what `/model` would show.
+  'codeMode:listModelOptions': {
+    req: z.object({ agent: CodingAgent }),
+    res: CodeAgentModelOptions,
+  },
   'codeSession:delete': {
     req: z.object({
       sessionId: z.string(),