Agents read several workspace files in one read_many_files call

Agents with the File System capability get a read_many_files tool. It reads 2 to 10 independent session-workspace files in one call, returns them in order with a per-file result or error, and stops below a 60 KiB combined output. read_file is unchanged.

Models that emit one tool call per response no longer need one model round per file. In the PR’s tests, three independent reads took 2 model calls instead of 4 and about half the input tokens. Loop detection recognizes repeated batch reads.

See File System and #3275.