Loki Mode

Hotfix: doctor install-hint bash/Bun parity (GH Bun Parity went red on v7.63.0)

v7.63.0 added the per-provider doctor install hint on the Bun route's STDOUT to
match bash. That matched the local-ci parity harness (which captures 2>&1) but
broke the canonical GH bun-parity gate, which captures STDOUT only
(>out 2>/dev/null): the bash route emits the hint to STDERR, so on a runner
with all providers absent the Bun route showed four extra stdout lines and the
gate went red. Fix: emit the hint on STDERR (matching bash run.sh >&2), so it
is invisible to the stdout-only gate and aligned under 2>&1. Added a regression
test that captures stdout only (mirroring the gate) and asserts the Bun route
emits no per-provider Install line on stdout. Verified parity under BOTH capture
modes (stdout-only and 2>&1).