The OpenAI service tier could only be chosen through the `tier.openai`
setting, or for a resumed session through whatever tier that session
recorded. Wanting flex or priority for a single run meant editing
settings and putting them back afterwards, while `bench` already took a
`--service-tier` flag that the session CLI did not offer.
Add `--service-tier` to the root command. The flag wins over the
configured setting and over a resumed session's recorded tier, leaves
the Anthropic and Google entries untouched, and records the resulting
map so a later resume keeps it. `none` removes the OpenAI entry, which
omits `service_tier` from the request.
Signed-off-by: Christian Stewart <christian@aperture.us>
