polish(cli): normalize usage-message casing to the clap house style

Every hand-rolled usage line now reads 'Usage: aura <verb> ...' — capital
prefix plus the program name, the shape clap itself and the built-in run
branch already spoke. 10 literal edits: 6 lowercase 'usage:' flips (graph
introspect, run blueprint x2, runs family, mc blueprint, mc built-in), 4 bare
closures gain the prefix (sweep/walkforward, blueprint + built-in branches);
the mc built-in line names the program in both |-separated alternatives.
Refusal diagnostics, the conforming run_args_from literal, clap output, and
the fieldtests corpus stay byte-identical; exit codes keep the usage=2 /
runtime=1 partition.

Lockstep pin: the single casing-sensitive assertion (mc --real) flips to
capital 'Usage'. Three additive regression tests pin the house style itself
(bare-closure prefix, both mc alternatives, mc blueprint branch) — the
pre-existing pins checked only verb tokens / a single Usage occurrence;
deviation from the spec's no-new-test-bytes wording recorded on the issue.

Verification: cargo test --workspace rc=0 (cli_run 133 passed), clippy -D
warnings clean, grep gates exact (zero lowercase 'usage:' literals; 11
'Usage: aura' source lines), smoke 'aura sweep --strategy bogus' prints the
house-style line and exits 2.

closes #179

crates/aura-cli/src/graph_construct.rs

@@ -188,7 +188,7 @@ pub fn introspect_cmd(cmd: crate::GraphIntrospectCmd) {
         + cmd.content_id as usize;
     if count != 1 {
         eprintln!(
-            "aura: usage: aura graph introspect --vocabulary | --node <T> | --unwired | --content-id"
+            "aura: Usage: aura graph introspect --vocabulary | --node <T> | --unwired | --content-id"
         );
         std::process::exit(2);
     }