feat: add executable production acceptance suite

[YangYuS8]

What changed

Deterministic security probes

• add nivora-probe with JSONL scenarios for service authentication, tenant isolation, anonymous Scope restrictions, Provider context requirements, unknown request fields, and history-role injection
• avoid storing response bodies or configured secrets in probe result artifacts
• add server-side acceptance tests for the same critical boundaries

Load and recovery tooling

• add nivora-load for bounded concurrent SSE testing
• record first-token and completion p50/p95/p99, success rate, and error distribution
• expose goroutine, heap allocation, and heap object metrics for post-load recovery checks
• add a synthetic Provider with deterministic knowledge, failed resources, matching charge/refund records, idempotent cases, latency injection, and first-N 429/5xx failures

Privacy-safe shadow comparison

• add nivora-shadow for baseline/candidate comparison
• evaluate the candidate against the existing deterministic regression cases
• store answer SHA-256, byte length, Tool sets, completion/error state, and latency instead of answer text
• replace request error details with generic codes so Provider responses or URLs are not persisted

Staging workflow and documentation

• add a manual Production Acceptance GitHub Actions workflow for protected nivora-staging environments
• add the executable production gate covering probes, regression, load/recovery, Shadow comparison, CozeLoop review, release metadata, and rollback evidence
• update English and Chinese README files and Make targets

Important boundary

This PR completes the repository-side acceptance tooling. It does not itself approve Lumio production traffic. Real acceptance still requires an isolated company staging environment, approved Ark/CozeLoop/VikingDB credentials, production-like PostgreSQL and infrastructure, synthetic or consented-redacted traffic, measured SLO/cost baselines, and a successful rollback drill.

Validation

CI will run:

• clean go mod tidy
• gofmt
• go vet ./...
• go test -race ./...
• go build -trimpath ./...

This builds Nivora and all evaluation, knowledge, probe, load, Shadow, and synthetic Provider commands.