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-arms.sh

@@ -48,7 +48,7 @@ fi
 
 LOG="$OUT/$ARM-r$RID.jsonl"; ERR="$OUT/$ARM-r$RID.err"
 ARGS=( -p "$Q" --output-format stream-json --verbose
-       --permission-mode bypassPermissions --model opus --max-budget-usd 4
+       --permission-mode bypassPermissions --model "${MODEL:-sonnet}" --effort "${EFFORT:-high}" --max-budget-usd 4
        --strict-mcp-config --mcp-config "$CFG" )
 [ -n "$STEERING" ] && ARGS+=( --append-system-prompt "$STEERING" )