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/run-agent.sh

@@ -25,7 +25,7 @@ cd "$REPO" || exit 1
 claude -p "$PROMPT" \
   --output-format stream-json --verbose \
   --permission-mode bypassPermissions \
-  --model opus \
+  --model "${MODEL:-sonnet}" --effort "${EFFORT:-high}" \
   --max-budget-usd 2 \
   --strict-mcp-config --mcp-config "$MCP_CONFIG" \
   > "$OUT" 2>"$OUT_DIR/run-${LABEL}.err"