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/bench-why-repo.sh

@@ -15,7 +15,7 @@ printf '{"mcpServers":{"codegraph":{"command":"%s","args":["serve","--mcp","--pa
 for i in $(seq 1 "$N"); do
   pkill -f "serve --mcp" 2>/dev/null; sleep 1; rm -f "$REPO/.codegraph/daemon.sock"
   ( cd "$REPO" && claude -p "$Q$WHY" --output-format stream-json --verbose \
-      --permission-mode bypassPermissions --model opus --effort "${EFFORT:-high}" --max-budget-usd 4 \
+      --permission-mode bypassPermissions --model "${MODEL:-sonnet}" --effort "${EFFORT:-high}" --max-budget-usd 4 \
       --strict-mcp-config --mcp-config "$OUT/cg.json" > "$OUT/with$i.jsonl" 2>"$OUT/with$i.err" )
   echo "WITH run $i: exit $? ($(wc -l < "$OUT/with$i.jsonl" | tr -d ' ') lines)"
 done