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/grep/constants.ts

@@ -113,8 +113,9 @@ export const DEFAULT_MAX_FILESIZE = "10M"
 export const DEFAULT_MAX_COUNT = 500
 export const DEFAULT_MAX_COLUMNS = 1000
 export const DEFAULT_CONTEXT = 2
-export const DEFAULT_TIMEOUT_MS = 300_000
-export const DEFAULT_MAX_OUTPUT_BYTES = 10 * 1024 * 1024
+export const DEFAULT_TIMEOUT_MS = 60_000
+export const DEFAULT_MAX_OUTPUT_BYTES = 256 * 1024
+export const DEFAULT_RG_THREADS = 4
 
 export const RG_SAFETY_FLAGS = [
   "--no-follow",