Settled design from #153 (fieldtest friction of milestone #148): make the
`aura run` cost-flag surface honest — a units/constraints note in the usage,
a named non-negativity diagnostic, and a refuse-don't-guess rejection of cost
flags on a non-R harness (the spec_gap; decided reject/exit-2, recorded on
#153). CLI-ergonomics + one C10 ledger note only; no cost-model/R-math change.
Boss-signed: grounding-check PASS (independent fresh-context agent; 6/6
load-bearing assumptions ratified by currently-green tests).
refs #153
Brummel
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