perf(session): reuse parsed journal during resume

Why:
SessionManager.open() parses the complete journal for its header and then
setSessionFile() parses the same journal again.

Changes:
- Reuse the entries already loaded by open() through a private setup path.
- Keep the public setSessionFile() contract unchanged.

Evidence:
- A 19.98 MB, 12,000-entry journal improved from 48.011 ms to
  25.406 ms median across 15 runs with identical restored state.

Refs #8117

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed session resume fully reading and parsing the journal twice by reusing the entries already loaded by `SessionManager.open()` ([#8117](https://github.com/can1357/oh-my-pi/issues/8117)).
+
 ## [17.2.12] - 2026-08-08
 
 ### Fixed

packages/coding-agent/src/session/session-manager.ts

@@ -1285,6 +1285,10 @@ export class SessionManager {
 
 	/** Switch to a different session file (resume / branch). */
 	async setSessionFile(sessionFile: string): Promise<void> {
+		await this.#setSessionFile(sessionFile);
+	}
+
+	async #setSessionFile(sessionFile: string, loadedEntries?: FileEntry[]): Promise<void> {
 		await this.#drainAndCloseWriter();
 		this.#clearDiskError();
 		this.#draftOnlySessionCleanupArmed = false;
@@ -1294,7 +1298,7 @@ export class SessionManager {
 		this.#rememberBreadcrumb(this.#cwd, resolvedSessionFile);
 
 		const titleSlot = await readTitleSlotFromFile(resolvedSessionFile, this.#storage);
-		const fileEntries = await loadEntriesFromFile(resolvedSessionFile, this.#storage);
+		const fileEntries = loadedEntries ?? (await loadEntriesFromFile(resolvedSessionFile, this.#storage));
 		if (fileEntries.length === 0) {
 			// Explicit but empty/missing path (e.g. --session flag): start fresh but
 			// keep the requested path and materialize the header immediately.
@@ -2601,7 +2605,7 @@ export class SessionManager {
 				: path.dirname(path.resolve(filePath)));
 		const manager = new SessionManager(cwd, dir, true, storage);
 		manager.#suppressBreadcrumb = options?.suppressBreadcrumb === true;
-		await manager.setSessionFile(filePath);
+		await manager.#setSessionFile(filePath, loaded);
 		return manager;
 	}
 

packages/coding-agent/test/session-manager-cwd-adoption.test.ts

@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, it } from "bun:test";
 import * as path from "node:path";
 import { SessionManager } from "@oh-my-pi/pi-coding-agent/session/session-manager";
+import { FileSessionStorage } from "@oh-my-pi/pi-coding-agent/session/session-storage";
 import { removeWithRetries, TempDir } from "@oh-my-pi/pi-utils";
 
 const tempDirs: TempDir[] = [];
@@ -115,18 +116,28 @@ describe("SessionManager cwd adoption on resume", () => {
 		expect(manager.getSessionDir()).toBe(path.resolve(launchSessions));
 	});
 
-	it("falls back to the launch cwd when opening a session whose project directory is gone", async () => {
+	it("falls back to the launch cwd with one full read when the recorded project directory is gone", async () => {
 		const launch = makeTempDir("@pi-cwd-launch-");
 		const store = makeTempDir("@pi-cwd-store-");
 		const goneProject = makeTempDir("@pi-cwd-gone-");
 		const file = await writeSession(goneProject, store);
 		await removeWithRetries(goneProject);
+		class CountingFileSessionStorage extends FileSessionStorage {
+			fullReads = 0;
 
-		const manager = await SessionManager.open(file, undefined, undefined, { initialCwd: launch });
+			override readText(filePath: string): Promise<string> {
+				this.fullReads++;
+				return super.readText(filePath);
+			}
+		}
+		const storage = new CountingFileSessionStorage();
+
+		const manager = await SessionManager.open(file, undefined, storage, { initialCwd: launch });
 
 		expect(manager.getCwd()).toBe(path.resolve(launch));
 		// /new and /branch anchor to the launch cwd, not the deleted project's store.
 		expect(manager.getSessionDir()).toBe(SessionManager.getDefaultSessionDir(launch));
 		expect(manager.getSessionDir()).not.toBe(path.resolve(store));
+		expect(storage.fullReads).toBe(1);
 	});
 });