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>

CLAUDE.md

@@ -137,6 +137,7 @@ For each **language × framework**, validate on **small, medium, and large** rea
 1. **Pick the canonical flow** for the framework ("how does X reach Y": state→render, request→handler→view, query→SQL, action→reducer→store…).
 2. **Deterministic probes** (`scripts/agent-eval/probe-{node,explore}.mjs` against the built `dist/`): `codegraph_explore` with the flow's symbol names connects from→to end-to-end with no break (its Flow section shows the path); **no node explosion** (`select count(*) from nodes` stable before/after re-index); synthesized-edge **precision** spot-check (`select … where provenance='heuristic'`).
 3. **Agent A/B** (`scripts/agent-eval/run-all.sh <repo> "<Q>"`): with vs without codegraph, **≥2 runs/arm** (run-to-run variance is large — never conclude from n=1). Record **duration, total tool calls, Read, Grep**. Optional forced-Read-0 sufficiency proof via the block-read hook (`scripts/agent-eval/hook-settings.json`).
+   - **Model policy — every A/B arm runs Claude with `--model sonnet --effort high`. Always. Never Opus/Fable.** All `scripts/agent-eval/*.sh` default to this (`MODEL`/`EFFORT` env override exists — don't raise it without an explicit reason from the maintainer). Two reasons, and the second matters more than cost: (a) Sonnet doesn't burn tokens; (b) **Sonnet is the deliberate floor model** — codegraph's real users attach it to whatever agent they already run (Cursor Composer, Gemini, etc.), so we validate on a "dumber" model on purpose: a stronger model's tool-use covers up the salience/sufficiency problems a weaker one exposes. An affordance that lands on Sonnet generalizes up to every host; one that only works on Opus/Fable doesn't generalize down to the agents most users actually have. Both arms always use the same model.
    - **MCP attach is a startup-latency issue, not a hard block.** On a multi-step task the agent dives into Read/grep before codegraph finishes its ~2-3s startup (worse when the eval is itself run nested inside a Claude session, under CPU contention), so it runs with no codegraph. Fix: **pre-warm a persistent daemon** for the target (`CODEGRAPH_DAEMON_IDLE_TIMEOUT_MS` high; spawn `serve --mcp --path <target> </dev/null &`; wait for `.codegraph/daemon.sock`) **and skip the startup re-exec** (`CODEGRAPH_WASM_RELAUNCHED=1`) so claude connects before the agent's first turn. Don't trust claude's `init` snapshot — it can read `status:"pending"` / 0 tools even when it then connects; judge by actual codegraph usage in `parse-run.mjs`'s `by type`. To isolate a change — **new-build vs baseline-build, both codegraph-on** (vs run-all.sh's with-vs-without) — use `scripts/agent-eval/ab-new-vs-baseline.sh <indexed-repo> "<task>" [baseline-ref]` (it bakes in the pre-warm).
 4. **Pass bar:** a normal flow question reaches **~0 Read/Grep within the repo's explore-call budget**, runs **faster** than without-codegraph, and shows **no regression on a control repo**. Record the numbers in `docs/design/dynamic-dispatch-coverage-playbook.md` (the coverage matrix).