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/index.ts

@@ -14,15 +14,18 @@ export { type ApiKeyCredential, type AuthCredential, AuthStorage, type OAuthCred
 export {
 	type BranchPreparation,
 	type BranchSummaryResult,
+	type CollectEntriesResult,
 	type CompactionResult,
 	type CutPointResult,
 	calculateContextTokens,
+	collectEntriesForBranchSummary,
 	compact,
 	DEFAULT_COMPACTION_SETTINGS,
 	estimateTokens,
 	type FileOperations,
 	findCutPoint,
 	findTurnStartIndex,
+	type GenerateBranchSummaryOptions,
 	generateBranchSummary,
 	generateSummary,
 	getLastAssistantUsage,