fix(acp): state the reply destination in mid-turn steer prompts

Justfile

@@ -340,6 +340,34 @@ test-unit:
         # `cargo test --workspace`; without this step a manifest edit that
         # diverges Rust from the corpus ships green.
         cargo nextest run -p buzz-agent --lib
+        # buzz-acp harness: ACP prompt formatting, the event queue, and the pool
+        # lifecycle state machine. Pure in-process tests, no infra. Enumerated
+        # explicitly because nothing in CI runs `cargo test --workspace`;
+        # without this step the steer prompt-body invariants (which thread a
+        # mid-turn message anchors the agent's reply to) compile but never
+        # execute.
+        #
+        # Clearing the environment is load-bearing. `config.rs` asserts clap
+        # defaults through `CliArgs::parse_from`, which reads `#[arg(env)]`
+        # variables unconditionally, and its test module deliberately avoids
+        # `std::env::set_var` to dodge parallelism races — so those tests assume
+        # a fixed environment. Inside a buzz-acp-hosted agent it is not fixed:
+        # the harness sets the very variables the package under test reads.
+        #
+        # The whole `BUZZ_*` family goes, not a list of names: clearing the
+        # family gives these tests a deterministic no-Buzz-config environment,
+        # where a name list only approximates one — and the list is a function of
+        # the test set, so it drifts in both directions as tests are added.
+        # `BUZZ_ACP_LAZY_POOL` and `BUZZ_ACP_MULTIPLE_EVENT_HANDLING` break
+        # default assertions; `BUZZ_ACP_ALLOWED_RESPOND_TO` breaks a test whose
+        # premise is that the option is unset. Subshell so the steps above keep
+        # their environment.
+        (
+            while IFS= read -r var; do
+                unset "$var"
+            done < <(env | sed -n 's/^\(BUZZ_[A-Za-z0-9_]*\)=.*/\1/p')
+            cargo nextest run -p buzz-acp
+        )
     else
         ./scripts/run-tests.sh unit
     fi