test(runner): the probe's home decision is pure, so its refusals are testable

/write-unit-test's diff ledger found one uncovered unit: homeWritable
had six negative branches and zero unit tests, reachable only through
the Linux-only integration probe.

Three of those six need no I/O at all — absent, empty, and relative
HOME are decidable from the value. Split out as homePathUsable, matching
the decider/probe shape preflight already uses ("deciders are pure
functions over already-fetched values so the absent-prerequisite
behavior is unit-testable without a deployment").

Absent is the branch that shipped: the child held a HOME the sandbox
never mounted, and nothing graded it, which is how all_ok=true stood on
a host where every lease died. Relative earns its own arm because the
probe would resolve it against the workspace and report a healthy home
that the engine never reads.

The three remaining branches are real I/O failures (bufPrint overflow,
create refused, delete refused) and stay with the integration probe.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>