Enhancement: Benchmark peek on a 10k+ file monorepo and document ceiling

[hariomlohardev]

Description

Dogfooding — peek caps at max_files=2000 but never benchmarks 10k+ monorepo. Document performance ceiling.

Files

• peek/tests/test_benchmark.py (new) — generate 10k files via tmp_path, time scan + analyze
• docs.md — add Performance section with 10k files: X sec, Y MB table

Acceptance

• Benchmark test exists and is pytest.mark.skip by default (slow)
• docs.md has 10k monorepo: ~2.1s etc.

Time

~60 min