Session tree: simplify types, add branching API, comprehensive tests

Types:
- SessionEntryBase with type field, extended by all entry types
- CustomEntry for hooks (type: 'custom', customType, data)
- Remove XXXContent types and TreeNode (redundant)

API:
- Rename saveXXX to appendXXX with JSDoc explaining tree semantics
- Rename branchInPlace to branch() with better docs
- Add createBranchedSession(leafId) replacing index-based version
- Add getTree() returning SessionTreeNode[] for tree traversal
- Add appendCustomEntry(customType, data) for hooks

Tests:
- tree-traversal.test.ts: 28 tests covering append, getPath, getTree,
  branch, branchWithSummary, createBranchedSession
- save-entry.test.ts: custom entry integration

Docs:
- Class-level JSDoc explaining append-only tree model
- Method docs explaining leaf advancement and branching
- CHANGELOG.md entry for all changes

packages/coding-agent/test/session-manager/save-entry.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from "vitest";
+import { type CustomEntry, SessionManager } from "../../src/core/session-manager.js";
+
+describe("SessionManager.saveCustomEntry", () => {
+	it("saves custom entries and includes them in tree traversal", () => {
+		const session = SessionManager.inMemory();
+
+		// Save a message
+		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+
+		// Save a custom entry
+		const customId = session.appendCustomEntry("my_hook", { foo: "bar" });
+
+		// Save another message
+		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,
+		});
+
+		// Custom entry should be in entries
+		const entries = session.getEntries();
+		expect(entries).toHaveLength(3);
+
+		const customEntry = entries.find((e) => e.type === "custom") as CustomEntry;
+		expect(customEntry).toBeDefined();
+		expect(customEntry.customType).toBe("my_hook");
+		expect(customEntry.data).toEqual({ foo: "bar" });
+		expect(customEntry.id).toBe(customId);
+		expect(customEntry.parentId).toBe(msgId);
+
+		// Tree structure should be correct
+		const path = session.getPath();
+		expect(path).toHaveLength(3);
+		expect(path[0].id).toBe(msgId);
+		expect(path[1].id).toBe(customId);
+		expect(path[2].id).toBe(msg2Id);
+
+		// buildSessionContext should work (custom entries skipped in messages)
+		const ctx = session.buildSessionContext();
+		expect(ctx.messages).toHaveLength(2); // only message entries
+	});
+});