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/03-brokerd-grant-mount-rule.md

@@ -42,7 +42,7 @@ text contains `cannot be mounted`. Other punctuation (space, `;`, `=`, `.`, `-`,
 - [ ] **4. See it pass.** `cargo test -p brokerd --test grants_mount --test grants`. Expected: 2 and
   17 passed.
 - [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
-- [ ] **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`
 
 ## Done when