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/glob/constants.ts

@@ -1,4 +1,4 @@
-export { resolveGrepCli, resolveGrepCliWithAutoInstall, type GrepBackend } from "../grep/constants"
+export { resolveGrepCli, resolveGrepCliWithAutoInstall, type GrepBackend, DEFAULT_RG_THREADS } from "../grep/constants"
 
 export const DEFAULT_TIMEOUT_MS = 60_000
 export const DEFAULT_LIMIT = 100