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/dirty-repo-guard.ts

@@ -5,13 +5,9 @@
  * Useful to ensure work is committed before switching context.
  */
 
-import type { HookAPI, HookEventContext } from "@mariozechner/pi-coding-agent/hooks";
+import type { HookAPI, HookContext } from "@mariozechner/pi-coding-agent/hooks";
 
-async function checkDirtyRepo(
-	pi: HookAPI,
-	ctx: HookEventContext,
-	action: string,
-): Promise<{ cancel: boolean } | undefined> {
+async function checkDirtyRepo(pi: HookAPI, ctx: HookContext, action: string): Promise<{ cancel: boolean } | undefined> {
 	// Check for uncommitted changes
 	const { stdout, code } = await pi.exec("git", ["status", "--porcelain"]);