feat(coding-agent/debug): Added interactive debug log viewer with selection controls

- Replaced the log preview with an interactive viewer supporting navigation, selection, expansion, and clipboard copy.
- Added log formatting helpers to wrap multi-line entries and parse timestamps for session boundary detection.
- Covered viewer selection, copy payload sanitization, expanded formatting, and timestamp parsing with new tests.

packages/coding-agent/test/debug/log-formatting.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, it } from "bun:test";
-import { formatDebugLogLine } from "../../src/debug/log-formatting";
+import {
+	formatDebugLogExpandedLines,
+	formatDebugLogLine,
+	parseDebugLogTimestampMs,
+} from "../../src/debug/log-formatting";
 
 describe("formatDebugLogLine", () => {
 	it("strips ANSI codes and carriage returns", () => {
@@ -26,4 +30,24 @@ describe("formatDebugLogLine", () => {
 		expect(Bun.stringWidth(result)).toBeLessThanOrEqual(10);
 		expect(result.startsWith("012345")).toBe(true);
 	});
+
+	it("wraps expanded log lines without dropping content", () => {
+		const input = "0123456789ABCDEFGHIJ";
+		const lines = formatDebugLogExpandedLines(input, 6);
+		expect(lines.length).toBeGreaterThan(1);
+		for (const line of lines) {
+			expect(Bun.stringWidth(line)).toBeLessThanOrEqual(6);
+		}
+	});
+
+	it("parses timestamp from JSON log lines", () => {
+		const input = '{"timestamp":"2026-02-14T12:34:56.000Z","level":"info","message":"ok"}';
+		expect(parseDebugLogTimestampMs(input)).toBe(Date.parse("2026-02-14T12:34:56.000Z"));
+	});
+
+	it("returns undefined when timestamp is missing or invalid", () => {
+		expect(parseDebugLogTimestampMs('{"message":"ok"}')).toBeUndefined();
+		expect(parseDebugLogTimestampMs('{"timestamp":"not-a-date"}')).toBeUndefined();
+		expect(parseDebugLogTimestampMs("not-json")).toBeUndefined();
+	});
 });

packages/coding-agent/test/debug/log-viewer.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it } from "bun:test";
+import { buildLogCopyPayload, DebugLogViewerModel, SESSION_BOUNDARY_WARNING } from "../../src/debug/log-viewer";
+
+describe("DebugLogViewerModel", () => {
+	it("inserts session boundary warning between older and current-session logs", () => {
+		const processStartMs = Date.parse("2026-02-14T12:00:00.000Z");
+		const logs = [
+			'{"timestamp":"2026-02-14T11:59:00.000Z","level":"info","message":"old"}',
+			'{"timestamp":"2026-02-14T11:59:30.000Z","level":"info","message":"old-2"}',
+			'{"timestamp":"2026-02-14T12:00:05.000Z","level":"info","message":"current"}',
+		].join("\n");
+
+		const model = new DebugLogViewerModel(logs, processStartMs);
+		const rowKinds = model.rows.map(row =>
+			row.kind === "warning" ? SESSION_BOUNDARY_WARNING : `log:${row.logIndex}`,
+		);
+
+		expect(rowKinds).toEqual(["log:0", "log:1", SESSION_BOUNDARY_WARNING, "log:2"]);
+	});
+
+	it("supports shift-range selection and reset on plain movement", () => {
+		const logs = ["a", "b", "c", "d"].join("\n");
+		const model = new DebugLogViewerModel(logs, Date.now());
+
+		model.moveCursor(1, true);
+		model.moveCursor(1, true);
+		expect(model.getSelectedLogIndices()).toEqual([0, 1, 2]);
+
+		model.moveCursor(1, false);
+		expect(model.getSelectedLogIndices()).toEqual([3]);
+	});
+
+	it("expands and collapses all selected rows", () => {
+		const logs = ["a", "b", "c"].join("\n");
+		const model = new DebugLogViewerModel(logs, Date.now());
+
+		model.moveCursor(1, true);
+		model.expandSelected();
+		expect(model.isExpanded(0)).toBe(true);
+		expect(model.isExpanded(1)).toBe(true);
+
+		model.collapseSelected();
+		expect(model.isExpanded(0)).toBe(false);
+		expect(model.isExpanded(1)).toBe(false);
+	});
+});
+
+describe("buildLogCopyPayload", () => {
+	it("joins selected lines and strips control/ansi sequences", () => {
+		const payload = buildLogCopyPayload(["plain", "\u001b[31mred\u001b[0m", "ok\u0007", ""]);
+		expect(payload).toBe("plain\nred\nok");
+	});
+});