Update codebase size summary in README

[associate-1]

Summary

• Update line counts to reflect current codebase (~14,700 → ~16,200)
• Production code: 6,400 → 7,300 lines
• Tests: 8,160 → 8,800 lines
• E2E test files: 5 files / 1,545 lines → 18 files / 3,826 lines

Test plan

• Verified line counts with wc -l across all .go files excluding kroc/

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