fix(compaction): add checkpoint store for session agent config

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/compaction-agent-config-checkpoint.ts

@@ -0,0 +1,42 @@
+export type CompactionAgentConfigCheckpoint = {
+  agent?: string
+  model?: { providerID: string; modelID: string }
+  tools?: Record<string, boolean>
+}
+
+const checkpoints = new Map<string, CompactionAgentConfigCheckpoint>()
+
+function cloneCheckpoint(
+  checkpoint: CompactionAgentConfigCheckpoint,
+): CompactionAgentConfigCheckpoint {
+  return {
+    ...(checkpoint.agent ? { agent: checkpoint.agent } : {}),
+    ...(checkpoint.model
+      ? {
+          model: {
+            providerID: checkpoint.model.providerID,
+            modelID: checkpoint.model.modelID,
+          },
+        }
+      : {}),
+    ...(checkpoint.tools ? { tools: { ...checkpoint.tools } } : {}),
+  }
+}
+
+export function setCompactionAgentConfigCheckpoint(
+  sessionID: string,
+  checkpoint: CompactionAgentConfigCheckpoint,
+): void {
+  checkpoints.set(sessionID, cloneCheckpoint(checkpoint))
+}
+
+export function getCompactionAgentConfigCheckpoint(
+  sessionID: string,
+): CompactionAgentConfigCheckpoint | undefined {
+  const checkpoint = checkpoints.get(sessionID)
+  return checkpoint ? cloneCheckpoint(checkpoint) : undefined
+}
+
+export function clearCompactionAgentConfigCheckpoint(sessionID: string): void {
+  checkpoints.delete(sessionID)
+}