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/examples/hooks/snake.ts

@@ -310,7 +310,7 @@ export default function (pi: HookAPI) {
 	pi.registerCommand("snake", {
 		description: "Play Snake!",
 
-		handler: async (ctx) => {
+		handler: async (_args, ctx) => {
 			if (!ctx.hasUI) {
 				ctx.ui.notify("Snake requires interactive mode", "error");
 				return;