feat(coding-agent): add get_entries and get_tree RPC commands

Adds two read-only RPC commands exposing existing SessionManager reads:

- get_entries: all session entries in append order, with optional since-entry-id cursor (strictly-after semantics, error on unknown id), plus current leafId.
- get_tree: getTree() roots plus current leafId.

Since sessions are append-only trees with stable entry ids, an entry id is a durable cursor: external orchestrators can use these commands to catch up after a restart without losing pre-compaction history, and can observe branch structure (/tree jumps, abandoned branches) that get_messages hides.

packages/coding-agent/src/index.ts

@@ -224,6 +224,7 @@ export {
 	type SessionInfoEntry,
 	SessionManager,
 	type SessionMessageEntry,
+	type SessionTreeNode,
 	type ThinkingLevelChangeEntry,
 } from "./core/session-manager.ts";
 export {