refactor(coding-agent): fix compaction for branched sessions, consolidate hook context types

Compaction API:
- prepareCompaction() now takes (pathEntries, settings) only
- CompactionPreparation restructured: removed cutPoint/messagesToKeep/boundaryStart, added turnPrefixMessages/isSplitTurn/previousSummary/fileOps/settings
- compact() now takes (preparation, model, apiKey, customInstructions?, signal?)
- Fixed token overflow by using getPath() instead of getEntries()

Hook types:
- HookEventContext renamed to HookContext
- HookCommandContext removed, RegisteredCommand.handler takes (args, ctx)
- HookContext now includes model field
- SessionBeforeCompactEvent: removed previousCompactions/model, added branchEntries
- SessionBeforeTreeEvent: removed model (use ctx.model)
- HookRunner.initialize() added for modes to set up callbacks

packages/coding-agent/test/compaction-hooks-example.test.ts

@@ -11,23 +11,27 @@ describe("Documentation example", () => {
 		const exampleHook = (pi: HookAPI) => {
 			pi.on("session_before_compact", async (event: SessionBeforeCompactEvent, ctx) => {
 				// All these should be accessible on the event
-				const { preparation, previousCompactions, model } = event;
-				// sessionManager and modelRegistry come from ctx, not event
-				const { sessionManager, modelRegistry } = ctx;
-				const { messagesToSummarize, messagesToKeep, tokensBefore, firstKeptEntryId, cutPoint } = preparation;
-
-				// Get previous summary from most recent compaction
-				const _previousSummary = previousCompactions[0]?.summary;
+				const { preparation, branchEntries, signal } = event;
+				// sessionManager, modelRegistry, and model come from ctx
+				const { sessionManager, modelRegistry, model } = ctx;
+				const {
+					messagesToSummarize,
+					turnPrefixMessages,
+					tokensBefore,
+					firstKeptEntryId,
+					isSplitTurn,
+					previousSummary,
+				} = preparation;
 
 				// Verify types
 				expect(Array.isArray(messagesToSummarize)).toBe(true);
-				expect(Array.isArray(messagesToKeep)).toBe(true);
-				expect(typeof cutPoint.firstKeptEntryIndex).toBe("number");
+				expect(Array.isArray(turnPrefixMessages)).toBe(true);
+				expect(typeof isSplitTurn).toBe("boolean");
 				expect(typeof tokensBefore).toBe("number");
-				expect(model).toBeDefined();
 				expect(typeof sessionManager.getEntries).toBe("function");
 				expect(typeof modelRegistry.getApiKey).toBe("function");
 				expect(typeof firstKeptEntryId).toBe("string");
+				expect(Array.isArray(branchEntries)).toBe(true);
 
 				const summary = messagesToSummarize
 					.filter((m) => m.role === "user")

packages/coding-agent/test/compaction-hooks.test.ts

@@ -143,12 +143,12 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 
 		const beforeEvent = beforeCompactEvents[0];
 		expect(beforeEvent.preparation).toBeDefined();
-		expect(beforeEvent.preparation.cutPoint.firstKeptEntryIndex).toBeGreaterThanOrEqual(0);
 		expect(beforeEvent.preparation.messagesToSummarize).toBeDefined();
-		expect(beforeEvent.preparation.messagesToKeep).toBeDefined();
+		expect(beforeEvent.preparation.turnPrefixMessages).toBeDefined();
 		expect(beforeEvent.preparation.tokensBefore).toBeGreaterThanOrEqual(0);
-		expect(beforeEvent.model).toBeDefined();
-		// sessionManager and modelRegistry are now on ctx, not event
+		expect(typeof beforeEvent.preparation.isSplitTurn).toBe("boolean");
+		expect(beforeEvent.branchEntries).toBeDefined();
+		// sessionManager, modelRegistry, and model are now on ctx, not event
 
 		const afterEvent = compactEvents[0];
 		expect(afterEvent.compactionEntry).toBeDefined();
@@ -363,19 +363,17 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 
 		expect(capturedBeforeEvent).not.toBeNull();
 		const event = capturedBeforeEvent!;
-		expect(event.preparation.cutPoint).toHaveProperty("firstKeptEntryIndex");
-		expect(event.preparation.cutPoint).toHaveProperty("isSplitTurn");
-		expect(event.preparation.cutPoint).toHaveProperty("turnStartIndex");
+		expect(typeof event.preparation.isSplitTurn).toBe("boolean");
+		expect(event.preparation.firstKeptEntryId).toBeDefined();
 
 		expect(Array.isArray(event.preparation.messagesToSummarize)).toBe(true);
-		expect(Array.isArray(event.preparation.messagesToKeep)).toBe(true);
+		expect(Array.isArray(event.preparation.turnPrefixMessages)).toBe(true);
 
 		expect(typeof event.preparation.tokensBefore).toBe("number");
 
-		expect(event.model).toHaveProperty("provider");
-		expect(event.model).toHaveProperty("id");
+		expect(Array.isArray(event.branchEntries)).toBe(true);
 
-		// sessionManager and modelRegistry are now on ctx, not event
+		// sessionManager, modelRegistry, and model are now on ctx, not event
 		// Verify they're accessible via session
 		expect(typeof session.sessionManager.getEntries).toBe("function");
 		expect(typeof session.modelRegistry.getApiKey).toBe("function");

packages/coding-agent/test/compaction.test.ts

@@ -11,6 +11,7 @@ import {
 	DEFAULT_COMPACTION_SETTINGS,
 	findCutPoint,
 	getLastAssistantUsage,
+	prepareCompaction,
 	shouldCompact,
 } from "../src/core/compaction/index.js";
 import {
@@ -398,12 +399,10 @@ describe.skipIf(!process.env.ANTHROPIC_OAUTH_TOKEN)("LLM summarization", () => {
 		const entries = loadLargeSessionEntries();
 		const model = getModel("anthropic", "claude-sonnet-4-5")!;
 
-		const compactionResult = await compact(
-			entries,
-			model,
-			DEFAULT_COMPACTION_SETTINGS,
-			process.env.ANTHROPIC_OAUTH_TOKEN!,
-		);
+		const preparation = prepareCompaction(entries, DEFAULT_COMPACTION_SETTINGS);
+		expect(preparation).toBeDefined();
+
+		const compactionResult = await compact(preparation!, model, process.env.ANTHROPIC_OAUTH_TOKEN!);
 
 		expect(compactionResult.summary.length).toBeGreaterThan(100);
 		expect(compactionResult.firstKeptEntryId).toBeTruthy();
@@ -421,12 +420,10 @@ describe.skipIf(!process.env.ANTHROPIC_OAUTH_TOKEN)("LLM summarization", () => {
 		const loaded = buildSessionContext(entries);
 		const model = getModel("anthropic", "claude-sonnet-4-5")!;
 
-		const compactionResult = await compact(
-			entries,
-			model,
-			DEFAULT_COMPACTION_SETTINGS,
-			process.env.ANTHROPIC_OAUTH_TOKEN!,
-		);
+		const preparation = prepareCompaction(entries, DEFAULT_COMPACTION_SETTINGS);
+		expect(preparation).toBeDefined();
+
+		const compactionResult = await compact(preparation!, model, process.env.ANTHROPIC_OAUTH_TOKEN!);
 
 		// Simulate appending compaction to entries by creating a proper entry
 		const lastEntry = entries[entries.length - 1];