Previously, we were not computing excerpt regions correctly for EP
examples captured from prod. This PR fixes that, and also simplifies the
data flow in the EP CLI. Examples either come from a concise spec (like
the markdown evals), or are collected from prod. Either way, we compute
from them a `ZetaPromptInput`, and the downstream steps like
prompt-formatting and scoring are derived from that.
Release Notes:
- N/A
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Co-authored-by: Ben Kunkle <ben@zed.dev>