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/bxctl/src/main.rs

@@ -4,11 +4,11 @@ use std::io::{BufRead, BufReader, Write};
 use std::path::{Path, PathBuf};
 use std::process::ExitCode;
 
+use bxcore::{ErrorCode, SessionId, Timestamp, TurnDone};
 use bxctl::admin::{self, AdminError};
 use bxctl::chat::{Approvals, ChatError, OnPending, Printer, TurnIo, new_session_id, stream_turn};
 use bxctl::cli::{self, ChatOptions, Command, USAGE};
 use bxctl::escape::escape_model_text;
-use proto::{ErrorCode, SessionId, Timestamp, TurnDone};
 
 fn main() -> ExitCode {
     // `args_os`, because `args` panics on an argument that is not UTF-8. bxctl's arguments are