test(ai): baseline 4 local CPU models + fix prompt-fairness bug

Capture native-harness benchmark baselines for qwen2.5 0.5b/1.5b/3b and
qwen2.5-coder:7b (all probed tool-capable; deepseek-r1 and NL2SH reject the
tools field). All cluster at 1-2/12 with high variance; the 7B buys no
pass-rate gain at ~3x latency, so qwen2.5:3b stays the default. The single
biggest score sink across every model is bare tool-call-as-text leaks — a
harness parse gap, the clear next improvement.

Also drop "in your home directory" from the shell prompts: it made literal-
minded models create a home/ subdir (/root/home/a/b/c/...) or use ~/, which
the benchmark itself surfaced. Findings doc + bench README carry the model
comparison table and recommendations (llama3.2:3b / llama3.1:8b for a non-qwen
data point).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bench/tasks.py

@@ -58,19 +58,19 @@ TASKS = [
     # ───────────────────────── shell ─────────────────────────
     Task(
         "shell-easy", "shell", "easy",
-        "run whoami and write its output to a file named who.txt in your home directory",
+        "run whoami and write its output to a file named who.txt",
         f'test "$(cat {HOME}/who.txt)" = root',
     ),
     Task(
         "shell-medium", "shell", "medium",
-        "create the nested directory a/b/c in your home directory and write the "
-        "text OK into a/b/c/marker.txt",
+        "create the nested directory a/b/c and write the text OK into "
+        "a/b/c/marker.txt",
         f'test "$(cat {HOME}/a/b/c/marker.txt)" = OK',
     ),
     Task(
         "shell-hard", "shell", "hard",
         "count how many files under /etc end in .conf and write just that number "
-        "to conf_count.txt in your home directory",
+        "to a file conf_count.txt",
         f'test "$(cat {HOME}/conf_count.txt)" = "$(find /etc -name "*.conf" -type f 2>/dev/null | wc -l | tr -d " ")"',
     ),