chore(agent-eval): standing A/B model policy — sonnet + high effort, never Opus/Fable (#816)

All agent A/B arms now run claude --model sonnet --effort high by default
(MODEL/EFFORT env overrides exist). Sonnet is the deliberate floor model:
codegraph's users attach whatever host they already run (Cursor Composer,
Gemini, ...), and a stronger model's tool-use masks the salience problems a
weaker one exposes — what lands on Sonnet generalizes up; Opus/Fable-only
wins don't generalize down. Policy recorded in CLAUDE.md's validation
methodology; 11 hardcoded --model opus call sites across 9 eval scripts
switched to the env-overridable default.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/agent-eval/ab-adoption.sh

@@ -71,7 +71,7 @@ run_arm() { # label, N
     echo "----- [$label] run $i -----"
     ( cd "$tgt" && claude -p "$TASK" \
         --output-format stream-json --verbose --permission-mode bypassPermissions \
-        --model opus --max-budget-usd 4 --strict-mcp-config --mcp-config "$c" \
+        --model "${MODEL:-sonnet}" --effort "${EFFORT:-high}" --max-budget-usd 4 --strict-mcp-config --mcp-config "$c" \
         </dev/null > "$OUT/run-$label-$i.jsonl" 2>"$OUT/run-$label-$i.err" )
     count "$OUT/run-$label-$i.jsonl"
     pkill -9 -f "serve --mcp --path $tgt" 2>/dev/null