perf(hooks,tools): optimize string operations and reduce redundant iterations

- output-renderer, hashline-edit-diff: replace str += with array join (H2)
- auto-slash-command: single-pass Map grouping instead of 6x filter (M1)
- comment-checker: hoist Zod schema to module scope (M2)
- session-last-agent: reverse iterate sorted array instead of sort+reverse (L2)

src/hooks/atlas/session-last-agent.ts

@@ -18,9 +18,9 @@ function getLastAgentFromMessageDir(messageDir: string): string | null {
     const files = readdirSync(messageDir)
       .filter((fileName) => fileName.endsWith(".json"))
       .sort()
-      .reverse()
 
-    for (const fileName of files) {
+    for (let i = files.length - 1; i >= 0; i--) {
+      const fileName = files[i]
       try {
         const content = readFileSync(join(messageDir, fileName), "utf-8")
         const parsed = JSON.parse(content) as { agent?: unknown }