test: test results and coverage reporting

[planetf1]

Context

Part of the Testing Infrastructure & Strategy Overhaul epic (item 8).

Objective

Surface test results and coverage data that is already generated but not visible anywhere. Capture metrics over time to enable trend reporting, flakiness detection, and coverage regression tracking.

Work

• Upload test results and coverage as CI artifacts (test: Add code coverage #352, feat: add code coverage tracking with pytest-cov #353, Move Code Coverage out of screen output #565) — data is generated but not surfaced
• Surface metrics: coverage data, skip counts, test durations, pass/fail rates
• Capture results over time to enable trend reporting and flakiness detection
• Detect coverage regressions between runs
• Update CI and contributor documentation

Notes

• This is about making existing data visible and useful, not generating new data
• Coverage metrics should exclude examples (per agreed principles)