Rename session from /resume session list (#863)

* Add session renaming in interactive mode resume picker

Session list now displays last message timestamp as modified time
instead of file mtime. Ctrl+N enters rename mode in the interactive
resume picker, allowing quick session renaming without leaving the
selector. Rename hint is shown only in interactive mode, not in the
CLI --resume picker./

* Add docs entry for renaming in picker

* Update shortcut to ctrl+r for session renaming

packages/coding-agent/test/session-info-modified-timestamp.test.ts

@@ -0,0 +1,83 @@
+import { writeFileSync } from "node:fs";
+import { stat } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeAll, describe, expect, it, vi } from "vitest";
+import type { SessionHeader } from "../src/core/session-manager.js";
+import { SessionManager } from "../src/core/session-manager.js";
+import { initTheme } from "../src/modes/interactive/theme/theme.js";
+
+function createSessionFile(path: string): void {
+	const header: SessionHeader = {
+		type: "session",
+		id: "test-session",
+		version: 3,
+		timestamp: new Date(0).toISOString(),
+		cwd: "/tmp",
+	};
+	writeFileSync(path, `${JSON.stringify(header)}\n`, "utf8");
+
+	// SessionManager only persists once it has seen at least one assistant message.
+	// Add a minimal assistant entry so subsequent appends are persisted.
+	const mgr = SessionManager.open(path);
+	mgr.appendMessage({
+		role: "assistant",
+		content: [{ type: "text", text: "hi" }],
+		api: "openai-completions",
+		provider: "openai",
+		model: "test",
+		usage: {
+			input: 1,
+			output: 1,
+			cacheRead: 0,
+			cacheWrite: 0,
+			totalTokens: 2,
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+		},
+		stopReason: "stop",
+		timestamp: Date.now(),
+	});
+}
+
+describe("SessionInfo.modified", () => {
+	beforeAll(() => initTheme("dark"));
+
+	afterEach(() => {
+		vi.restoreAllMocks();
+	});
+
+	it("uses last user/assistant message timestamp instead of file mtime", async () => {
+		const filePath = join(tmpdir(), `pi-session-${Date.now()}-modified.jsonl`);
+		createSessionFile(filePath);
+
+		const before = await stat(filePath);
+		// Ensure the file mtime can differ from our message timestamp even on coarse filesystems.
+		await new Promise((r) => setTimeout(r, 10));
+
+		const mgr = SessionManager.open(filePath);
+		const msgTime = Date.now();
+		mgr.appendMessage({
+			role: "assistant",
+			content: [{ type: "text", text: "later" }],
+			api: "openai-completions",
+			provider: "openai",
+			model: "test",
+			usage: {
+				input: 1,
+				output: 1,
+				cacheRead: 0,
+				cacheWrite: 0,
+				totalTokens: 2,
+				cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+			},
+			stopReason: "stop",
+			timestamp: msgTime,
+		});
+
+		const sessions = await SessionManager.list("/tmp", filePath.replace(/\/[^/]+$/, ""));
+		const s = sessions.find((x) => x.path === filePath);
+		expect(s).toBeDefined();
+		expect(s!.modified.getTime()).toBe(msgTime);
+		expect(s!.modified.getTime()).not.toBe(before.mtime.getTime());
+	});
+});