Add label support for session entries

- Add LabelEntry type with targetId and label (string | undefined)
- Add labelsById map built on load via linear scan
- Add getLabel(id) and appendLabelChange(targetId, label) methods
- Add label field to SessionTreeNode, populated by getTree()
- Update createBranchedSession to preserve labels for entries on path
- Labels are ignored by buildSessionContext (not sent to LLM)
- Add comprehensive tests for label functionality

packages/coding-agent/test/session-manager/labels.test.ts

@@ -0,0 +1,178 @@
+import { describe, expect, it } from "vitest";
+import { type LabelEntry, SessionManager } from "../../src/core/session-manager.js";
+
+describe("SessionManager labels", () => {
+	it("sets and gets labels", () => {
+		const session = SessionManager.inMemory();
+
+		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+
+		// No label initially
+		expect(session.getLabel(msgId)).toBeUndefined();
+
+		// Set a label
+		const labelId = session.appendLabelChange(msgId, "checkpoint");
+		expect(session.getLabel(msgId)).toBe("checkpoint");
+
+		// Label entry should be in entries
+		const entries = session.getEntries();
+		const labelEntry = entries.find((e) => e.type === "label") as LabelEntry;
+		expect(labelEntry).toBeDefined();
+		expect(labelEntry.id).toBe(labelId);
+		expect(labelEntry.targetId).toBe(msgId);
+		expect(labelEntry.label).toBe("checkpoint");
+	});
+
+	it("clears labels with undefined", () => {
+		const session = SessionManager.inMemory();
+
+		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+
+		session.appendLabelChange(msgId, "checkpoint");
+		expect(session.getLabel(msgId)).toBe("checkpoint");
+
+		// Clear the label
+		session.appendLabelChange(msgId, undefined);
+		expect(session.getLabel(msgId)).toBeUndefined();
+	});
+
+	it("last label wins", () => {
+		const session = SessionManager.inMemory();
+
+		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+
+		session.appendLabelChange(msgId, "first");
+		session.appendLabelChange(msgId, "second");
+		session.appendLabelChange(msgId, "third");
+
+		expect(session.getLabel(msgId)).toBe("third");
+	});
+
+	it("labels are included in tree nodes", () => {
+		const session = SessionManager.inMemory();
+
+		const msg1Id = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+		const msg2Id = session.appendMessage({
+			role: "assistant",
+			content: [{ type: "text", text: "hi" }],
+			api: "anthropic-messages",
+			provider: "anthropic",
+			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: 2,
+		});
+
+		session.appendLabelChange(msg1Id, "start");
+		session.appendLabelChange(msg2Id, "response");
+
+		const tree = session.getTree();
+
+		// Find the message nodes (skip label entries)
+		const msg1Node = tree.find((n) => n.entry.id === msg1Id);
+		expect(msg1Node?.label).toBe("start");
+
+		// msg2 is a child of msg1
+		const msg2Node = msg1Node?.children.find((n) => n.entry.id === msg2Id);
+		expect(msg2Node?.label).toBe("response");
+	});
+
+	it("labels are preserved in createBranchedSession", () => {
+		const session = SessionManager.inMemory();
+
+		const msg1Id = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+		const msg2Id = session.appendMessage({
+			role: "assistant",
+			content: [{ type: "text", text: "hi" }],
+			api: "anthropic-messages",
+			provider: "anthropic",
+			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: 2,
+		});
+
+		session.appendLabelChange(msg1Id, "important");
+		session.appendLabelChange(msg2Id, "also-important");
+
+		// Branch from msg2 (in-memory mode returns null, but updates internal state)
+		session.createBranchedSession(msg2Id);
+
+		// Labels should be preserved
+		expect(session.getLabel(msg1Id)).toBe("important");
+		expect(session.getLabel(msg2Id)).toBe("also-important");
+
+		// New label entries should exist
+		const entries = session.getEntries();
+		const labelEntries = entries.filter((e) => e.type === "label") as LabelEntry[];
+		expect(labelEntries).toHaveLength(2);
+	});
+
+	it("labels not on path are not preserved in createBranchedSession", () => {
+		const session = SessionManager.inMemory();
+
+		const msg1Id = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+		const msg2Id = session.appendMessage({
+			role: "assistant",
+			content: [{ type: "text", text: "hi" }],
+			api: "anthropic-messages",
+			provider: "anthropic",
+			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: 2,
+		});
+		const msg3Id = session.appendMessage({ role: "user", content: "followup", timestamp: 3 });
+
+		// Label all messages
+		session.appendLabelChange(msg1Id, "first");
+		session.appendLabelChange(msg2Id, "second");
+		session.appendLabelChange(msg3Id, "third");
+
+		// Branch from msg2 (excludes msg3)
+		session.createBranchedSession(msg2Id);
+
+		// Only labels for msg1 and msg2 should be preserved
+		expect(session.getLabel(msg1Id)).toBe("first");
+		expect(session.getLabel(msg2Id)).toBe("second");
+		expect(session.getLabel(msg3Id)).toBeUndefined();
+	});
+
+	it("labels are not included in buildSessionContext", () => {
+		const session = SessionManager.inMemory();
+
+		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+		session.appendLabelChange(msgId, "checkpoint");
+
+		const ctx = session.buildSessionContext();
+		expect(ctx.messages).toHaveLength(1);
+		expect(ctx.messages[0].role).toBe("user");
+	});
+
+	it("throws when labeling non-existent entry", () => {
+		const session = SessionManager.inMemory();
+
+		expect(() => session.appendLabelChange("non-existent", "label")).toThrow("Entry non-existent not found");
+	});
+});