Add ReadonlySessionManager and refactor branch summarization

- Add ReadonlySessionManager interface to session-manager.ts
- Re-export from hooks/index.ts
- Add collectEntriesForBranchSummary() to extract entries for summarization
- Don't stop at compaction boundaries (include their summaries as context)
- Add token budget support to prepareBranchEntries()
- Walk entries newest-to-oldest to prioritize recent context
- Use options object for generateBranchSummary()
- Handle compaction entries as context summaries
- Export new types: CollectEntriesResult, GenerateBranchSummaryOptions

packages/coding-agent/src/core/session-manager.ts

@@ -155,6 +155,20 @@ export interface SessionInfo {
 	allMessagesText: string;
 }
 
+/**
+ * Read-only interface for SessionManager.
+ * Used by compaction/summarization utilities that only need to read session data.
+ */
+export interface ReadonlySessionManager {
+	getLeafId(): string | null;
+	getEntry(id: string): SessionEntry | undefined;
+	getPath(fromId?: string): SessionEntry[];
+	getEntries(): SessionEntry[];
+	getChildren(parentId: string): SessionEntry[];
+	getTree(): SessionTreeNode[];
+	getLabel(id: string): string | undefined;
+}
+
 /** Generate a unique short ID (8 hex chars, collision-checked) */
 function generateId(byId: { has(id: string): boolean }): string {
 	for (let i = 0; i < 100; i++) {