M3b plan: every task's git add includes Cargo.lock

Task 04 added dependencies to toolkit and its git add line left out the lock
file, so the driver stopped on an unclean tree. The lock change is folded into
task 04's commit.

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

docs/plans/M3b/13-brokerd-serve-runner.md

@@ -43,7 +43,7 @@ there today). Print the notice with `eprintln!` right after the existing
   and 9 passed.
 - [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`, with about 638 tests in
   all.
-- [ ] **6. Log and commit.** `git add crates/brokerd docs/implementer-log.md && git commit`
+- [ ] **6. Log and commit.** `git add crates/brokerd docs/implementer-log.md Cargo.lock && git commit`
 
 This is the last task of M3b. Stop after the commit; the review comes next.