benchmark: expand the public corpus with one new repository slice

[vishal2612200]

Goal

Add one new public repository slice to the benchmark corpus without overfitting rankings to that repository.

Suggested files

• benchmarks/public-repos.toml
• benchmarks/results/YYYY-MM-DD-public.md after running the suite
• docs/benchmark-learnings.md if the slice changes tuning decisions

Acceptance criteria

• Adds real historical tasks with expected files.
• Runs the public benchmark command and records the dated result.
• Documents any ranking changes as generalizable rules, not repo-specific hacks.