Test Discipline: Replace Binary with Three-Way Classification (High Priority)

[kenjudy]

The current metric collapses three very different commit types into a binary:

Commit Type	Current Classification	Reality
Test + Prod files	test_first_indicator: true	Positive (co-discipline)
Test files only	test_first_indicator: false	Positive (TDD red phase, test improvement)
Prod files only, large	test_first_indicator: false	Critical negative (code without coverage)

A commit that only improves tests is currently indistinguishable from a commit that adds 200 lines of production code with no tests at all. Both score test_first_indicator: false.

Proposed replacement metrics:

test_coverage_rate:    (commits with test AND prod) / total
test_isolation_rate:   (commits with test ONLY) / total  
uncovered_prod_rate:   (commits with prod ONLY, large) / total

uncovered_prod_rate on large commits is the highest-risk signal for AI drift — exactly the "accepted AI output without writing tests" pattern. This is currently invisible.