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/src/core/index.ts

@@ -27,9 +27,9 @@ export {
 } from "./custom-tools/index.js";
 export {
 	type HookAPI,
+	type HookContext,
 	type HookError,
 	type HookEvent,
-	type HookEventContext,
 	type HookFactory,
 	HookRunner,
 	type HookUIContext,