Build minimal LLM eval harness scaffold (B5) — fixtures recorded separately

Land the B5 harness skeleton: fixture loader (strict JSONL contract with
meta header + ordered turn records + assertions tail), replay-client that
swaps in for pi-ai's streamSimple by replaying recorded assistant turns,
structural assertion judge (tool_called / no_tool_called /
message_contains_substring / tool_call_arg_matches / trace_max_turns), an
eval-runner that sums the token ledger and produces an EvalResult, a
vitest driver (tests/eval/harness.test.ts) that discovers *.jsonl
fixtures, and a record-fixture CLI that writes fixture meta headers.

Hard contracts:
- No fallback: replay-client and fixture-loader throw on exhaustion /
  malformed input rather than synthesizing defaults.
- No mock: types reuse @mariozechner/pi-ai AgentMessage / ToolCall
  verbatim; shadow types are forbidden.
- No fake fixture data: tests/eval/fixtures/ ships only README +
  .gitkeep. Real traces are recorded by the user against live providers.

package.json adds test:eval (vitest --passWithNoTests so an empty
fixture dir is green) and eval:record (CLI entrypoint). electron-builder
already excludes tests/ via its files: glob — fixtures never enter the
shipped .asar.

tests/eval/fixtures/README.md

@@ -0,0 +1,44 @@
+# Eval fixtures
+
+Every file in this directory is a **recorded trace** of a real AgentCompany
+turn against a real LLM provider. Fixtures are recorded-truth, not
+synthesized mock data — the distinction is what keeps the B5 harness on the
+"fixture, not fallback" side of the roundtable line.
+
+## File format
+
+Each fixture is a `.jsonl` file with one JSON object per line:
+
+1. First line: a `{ "type": "meta", ... }` record (see `FixtureMeta` in
+   `../types.ts`).
+2. Middle lines: one `{ "type": "turn", "turnIndex": N, ... }` record per
+   conversation turn, in order. `turnIndex` is strictly increasing from `0`.
+3. Last line: a `{ "type": "assertions", "assertions": [...] }` record with
+   the expected behavior contract.
+
+The loader rejects files that do not match this shape. No silent skipping.
+
+## Recording workflow
+
+```bash
+pnpm eval:record -- \
+  --fixture-id ceo-team-building-empty \
+  --scenario "Empty company, CEO bootstrap" \
+  --out tests/eval/fixtures/ceo-team-building-empty.jsonl \
+  --system-prompt-preview "$(cat /tmp/current-system-prompt.txt)"
+```
+
+The current CLI writes a stub meta + empty assertions. The full recorder
+(which drives `pi-agent-core` against a real provider using the
+AgentCompany worker's system prompt and tool registry) lands in a
+follow-up commit — see `../README.md` for the B5 roadmap.
+
+## Committing fixtures
+
+Fixtures belong in git. They are small, deterministic, and the whole point
+of the eval tier is to diff behavior against recorded truth. Do NOT add
+`tests/eval/fixtures/` to `.gitignore`.
+
+Fixtures are excluded from the electron-builder production bundle; see
+`electron-builder.yml` (the `files` glob only picks `out/**`, so `tests/`
+never enters the packaged `.asar`).