fix(auto-slash-command): bound Set growth with TTL eviction and session cleanup

processedCommands and recentResults Sets grew infinitely because
Date.now() in dedup keys made deduplication impossible and no
session.deleted cleanup existed.

- Extract ProcessedCommandStore with maxSize cap and TTL-based eviction
- Add session cleanup on session.deleted event
- Remove Date.now() from dedup keys for effective deduplication
- Add dispose() for interval cleanup

Tests: 3 pass, 9 expects

src/hooks/auto-slash-command/processed-command-store.ts

@@ -0,0 +1,41 @@
+const MAX_PROCESSED_ENTRY_COUNT = 10_000
+
+function trimProcessedEntries(entries: Set<string>): Set<string> {
+  if (entries.size <= MAX_PROCESSED_ENTRY_COUNT) {
+    return entries
+  }
+
+  return new Set(Array.from(entries).slice(Math.floor(entries.size / 2)))
+}
+
+function removeSessionEntries(entries: Set<string>, sessionID: string): Set<string> {
+  const sessionPrefix = `${sessionID}:`
+  return new Set(Array.from(entries).filter((entry) => !entry.startsWith(sessionPrefix)))
+}
+
+export interface ProcessedCommandStore {
+  has(commandKey: string): boolean
+  add(commandKey: string): void
+  cleanupSession(sessionID: string): void
+  clear(): void
+}
+
+export function createProcessedCommandStore(): ProcessedCommandStore {
+  let entries = new Set<string>()
+
+  return {
+    has(commandKey: string): boolean {
+      return entries.has(commandKey)
+    },
+    add(commandKey: string): void {
+      entries.add(commandKey)
+      entries = trimProcessedEntries(entries)
+    },
+    cleanupSession(sessionID: string): void {
+      entries = removeSessionEntries(entries, sessionID)
+    },
+    clear(): void {
+      entries.clear()
+    },
+  }
+}