feat(test): pluggable LLM agent for live-mode E2E suite

tests/live-e2e/agents/llm-agent.mjs: a Claude-backed VariantAgent that
implements the same one-method interface as the fake agent
(generateVariants(event, context) → { scopedCss, variants[] }). Default
model claude-haiku-4-5; override via IMPECCABLE_E2E_LLM_MODEL.

Prompt caching is on — the system prompt (instructions + the live-mode
spec from reference/live.md) is the cacheable prefix. First call writes
~10K tokens to cache; subsequent fixtures pay only the cache-read rate.
JSON output is validated for shape (scopedCss, variants[N].innerHtml),
with light error messages on parse failure.

tests/live-e2e.test.mjs: read IMPECCABLE_E2E_AGENT (fake|llm). When 'llm',
construct the LLM agent and skip the case cleanly if ANTHROPIC_API_KEY is
unset. Param-manifest assertions are gated to fake mode (LLM may emit
zero-param "fixed point" variants per the live.md spec). The accepted-h1
class assertion now allows hero-title as one of multiple classes so an
LLM agent that adds classes alongside the original still passes.

Test timeouts widen for LLM mode: 25s first-pass on conditional-render
fixtures (vs 5s for fake), 60s on direct waits (vs 30s). Without these,
the LLM's 3-8s generate latency races the orchestration's state-loss
recovery window.

tests/live-e2e/ui.mjs: clickGo retries up to 3× on stability failures.
Required because conditional-render fixtures (modal/tabs) animate the bar
mid-transition when preActions trigger framework HMR; a single click can
land during a re-render and Playwright's stability gate times out.

Pass rate on a typical sweep: 18/19 in LLM mode, 19/19 in fake mode.
The modal fixture's intrinsic state-loss flake (Fast Refresh resetting
useState(open) when source changes) is amplified by LLM latency and may
need a re-run; documented in CLAUDE.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -126,7 +126,9 @@ IMPECCABLE_E2E_DEBUG=1 bun run test:live-e2e                # dump page DOM + de
 
 **Kept out of the default `bun run test`** because (a) it does real `npm install` per fixture, (b) it boots framework dev servers, (c) wall time is ~2 minutes, and (d) it requires Playwright's browser cache. Run it locally before shipping changes to anything in `source/skills/impeccable/scripts/live-*.{mjs,js}`.
 
-The agent is pluggable via a one-method interface in `tests/live-e2e/agent.mjs`: `generateVariants(event, context) → { scopedCss, variants[] }`. The default fake agent emits canned variants that exercise all three param kinds (`range`, `steps`, `toggle`). A future LLM-backed agent slots in by implementing the same shape; the orchestrator (wrap, write, accept, carbonize) is agent-agnostic.
+The agent is pluggable via a one-method interface in `tests/live-e2e/agent.mjs`: `generateVariants(event, context) → { scopedCss, variants[] }`. The default fake agent emits canned variants that exercise all three param kinds (`range`, `steps`, `toggle`). The orchestrator (wrap, write, accept, carbonize) is agent-agnostic.
+
+**LLM agent (opt-in)**: set `IMPECCABLE_E2E_AGENT=llm` to swap the fake agent for `tests/live-e2e/agents/llm-agent.mjs`, which calls Claude (default Haiku 4.5) via `@anthropic-ai/sdk`. Requires `ANTHROPIC_API_KEY` in env; the test runner skips with a clear message when it's unset. Override the model with `IMPECCABLE_E2E_LLM_MODEL=claude-sonnet-4-6` if Haiku produces unreliable JSON. Caching is on — live.md is the cacheable prefix, and after the first call subsequent fixtures pay only the cache-read rate. Pass rate on a typical sweep is 18/19; the modal fixture's intrinsic state-loss flake is amplified by LLM latency and may need a re-run. **This path hits the API and costs money** — keep it out of CI unless you really want it there.
 
 Adding a new fixture is a matter of cloning a directory under `tests/framework-fixtures/`, swapping the source files, and writing a `fixture.json`. See `tests/framework-fixtures/README.md` for the full schema.