Mask secret values in the log as it is written

Mask secret values in the log as it is written

A step that prints a secret by accident used to put it in the attempt log, and
the log is streamed to the instance and kept. The runner now replaces every
value it was given with *** before the bytes reach the log file, so the copy
the server stores never holds one and a value split across two writes is still
caught: the mask holds back the last longest-1 bytes and the runner flushes at
the end of each step.

ci/redact is the one implementation, so a local `vole workflow run` masks its
terminal output the same way. Its own error text goes through the same mask,
because "the runtime message does not contain a value" should be true rather
than assumed.

It is best effort, and the docs say where it stops: a re-encoded value does not
match, a value shorter than four bytes is left alone because masking every
occurrence of it would rewrite the log, and an artifact or a site file is
published exactly as the step wrote it.

Slice 3 of #154.