Replace the hand-rolled aura-cli argv parser with a clap derive parser:
scoped `aura <sub> --help`, `--version`/`-V`, a per-flag Options section, and
`--flag=value`/`--`/long-opt abbreviation for free. Two iterations — (1) clap
migration, behaviour-preserving on exit codes; (2) exit-code split (usage=2,
runtime=1). clap is admitted under the C16 per-case review (research-side CLI,
invariant 8 untouched).
Forks ratified by the user (clap over hand-rolled; GNU-compliance over the
machine-first reframe; exit-code Option A) and the three spec-time derived
decisions (dual-grammar clap mapping, parse-vs-runtime exit classification,
two-iteration split) are recorded on #175.
Boss-signed on grounding-check PASS (independent, fresh-context; 13
current-behaviour assumptions all ratified by named green tests).
refs #175
