proto becomes bxcore, the one shared crate (M5b task 00)

Done the way the owner asked and the spike mapped: the crate and the
manifests renamed, then what the build reported fixed. Layers: 138
"proto unresolved" errors across 169 files; cargo fmt reordering
imports; six fixture paths built as strings in brokerd's and bxctl's
tests, which the compiler cannot see, and comments naming
crates/proto/; the crate-dependency check and its self-test. One more
the spike did not reach: a `\nproto =` key inside a printf format in
the self-test, which a word-boundary replacement skips because `n` and
`p` are both word characters.

make gate: gate: ok, 944 tests; no new entry in /tmp.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/loopd/tests/request.rs

@@ -83,7 +83,7 @@ fn thinking_off_and_tools() {
 /// here on purpose: they are rendered differently.
 #[test]
 fn every_kind_of_message() {
-    let call = proto::ToolCall {
+    let call = bxcore::ToolCall {
         id: "call_1".to_string(),
         name: "read_file".to_string(),
         arguments: r#"{"path":"/etc/hostname"}"#.to_string(),