ci(go): fix coverage gate by instrumenting all packages with -coverpkg

[izo]

Summary

• The Go coverage gate in .github/workflows/test.yml ran go test -coverprofile=coverage.out ./... without -coverpkg=./.... Each package's profile only tracked its own statements; cross-package execution (e.g. cmd integration tests driving internal/installer, internal/registry, tui) was never attributed, and packages without their own tests were absent from the profile entirely. The 70% threshold was passing on artificially inflated per-package self-coverage.
• Add -coverpkg=./... so every package is instrumented on every test run and cross-package usage is properly counted.
• Drop the threshold from 70% → 35%. With honest measurement, real coverage surfaces at ~38.8%; 35% leaves a small margin for normal fluctuation while still acting as a regression guard.
• Remove the 2>/dev/null that was hiding go test stderr.

Test plan

• Run go test -coverpkg=./... -coverprofile=coverage.out ./... locally → all package tests pass.
• go tool cover -func=coverage.out | tail -1 → total: (statements) 38.8%.
• Threshold check (pct=38; [ "$pct" -lt 35 ] → false) → gate passes.
• CI run on this PR confirms the new gate executes cleanly across ubuntu-latest and macos-latest.

https://claude.ai/code/session_016UwwMWsDgFY16GHiEAwuvu

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