Split the two jobs SourceConfig.est_tokens has been doing: TPM entry
pre-deduction, where conservative means safe, and the telemetry usage
fallback, where feeding a worst-case upper bound through the output
price inflates cost by ~26x.
Records the approved decisions: usage_source gains an "unavailable"
state whose cost is NULL, and an unset est_tokens derives from
tpm//60 so the in-flight ceiling stays scale-invariant. Also declares
the CHS "conservative accounting" migration item as intentionally
dropped, and the pre-existing global-TPM gap as knowingly unfixed.
Refs: gitea issue #2
