fix(tools): throttle ripgrep CPU usage with thread limits and concurrency control

- Add --threads=4 flag to all rg invocations (grep and glob)
- Add global semaphore limiting concurrent rg processes to 2
- Reduce grep timeout from 300s to 60s (matches tool description)
- Reduce max output from 10MB to 256KB (prevents excessive memory usage)
- Add output_mode parameter (content/files_with_matches/count)
- Add head_limit parameter for incremental result fetching

Closes #2008

Ref: #674, #1722

src/tools/shared/semaphore.ts

@@ -0,0 +1,32 @@
+/**
+ * Simple counting semaphore to limit concurrent process execution.
+ * Used to prevent multiple ripgrep processes from saturating CPU.
+ */
+export class Semaphore {
+  private queue: (() => void)[] = []
+  private running = 0
+
+  constructor(private readonly max: number) {}
+
+  async acquire(): Promise<void> {
+    if (this.running < this.max) {
+      this.running++
+      return
+    }
+    return new Promise<void>((resolve) => {
+      this.queue.push(() => {
+        this.running++
+        resolve()
+      })
+    })
+  }
+
+  release(): void {
+    this.running--
+    const next = this.queue.shift()
+    if (next) next()
+  }
+}
+
+/** Global semaphore limiting concurrent ripgrep processes to 2 */
+export const rgSemaphore = new Semaphore(2)