feat(compaction): add proactive context budget and checkpointing

src/context/compaction.ts

@@ -15,6 +15,23 @@ export interface CompactionConfig {
   summaryMaxTokens: number;
   /** Preserve messages at or above this importance score from compaction. */
   importanceThreshold: number;
+  /** Optional proactive context usage thresholds and actions. */
+  proactive?: ProactiveCompactionConfig;
+}
+
+export interface ProactiveCompactionConfig {
+  /** Enable proactive context warnings/checkpoints before hard compaction cliffs. */
+  enabled: boolean;
+  /** Emit warning signals when usage crosses this percentage. */
+  warnPct: number;
+  /** Save a checkpoint summary to memory when usage crosses this percentage. */
+  checkpointPct: number;
+  /** Auto-run compaction when usage crosses this percentage. */
+  autoCompactPct: number;
+  /** Cooldown window between checkpoint writes. */
+  checkpointCooldownMs: number;
+  /** Memory namespace base for proactive checkpoints. */
+  memoryNamespace: string;
 }
 
 export interface CompactionResult {
@@ -33,6 +50,14 @@ export const DEFAULT_COMPACTION_CONFIG: CompactionConfig = {
   keepTurns: 4,
   summaryMaxTokens: 1024,
   importanceThreshold: 1,
+  proactive: {
+    enabled: false,
+    warnPct: 75,
+    checkpointPct: 85,
+    autoCompactPct: 95,
+    checkpointCooldownMs: 300_000,
+    memoryNamespace: 'session/checkpoints',
+  },
 };
 
 export async function compactHistory(opts: {