Move agent file benchmark into the public repo

[nambok]

Summary

The blog post points readers at the benchmark, but it lived in the private platform repository. This moves the whole suite (runners, fixtures, parse caches, rules, tasks) here so anyone can rerun it.

Changes

• benchmarks/agent_file: two_pass.py, sweep.py, fast_tune.py, helpers, six fixtures with rules and cached parses
• benchmarks/agent_file/README.md: what each piece is and how to run it
• fixtures/SOURCES.md: upstream attribution for the four public files
• fast_tune.py: AWS profile comes from the environment, no personal default
• README.md: benchmark pointer now references this repository

Verification

• Secret scan over the moved files: only placeholder text from the public fixtures themselves
• sweep.py is offline; runner code unchanged apart from the profile default