Document batch tracing fields in README telemetry list

[hardbyte]

Context

The v0.3 README's Tracing And Telemetry section says batch evaluation records aggregate and per-policy fields, but the explicit emitted-field bullet list only names the single-item gatehouse::security event fields.

The prose mentions these batch fields, but the enumerated list should lock them in too:

• item_count
• granted_count
• denied_count
• policy_count
• max_batch_size
• policy.pending_count
• policy.granted_count
• policy.denied_count
• policy.type on the nested gatehouse.batch_policy span

Why

Operators and reviewers will copy the field list when building tracing queries. If the list omits batch fields, the README under-documents one of the main v0.3 audit/debugging improvements.

Scope

Docs-only follow-up. Do not change tracing behavior unless this reveals a mismatch between docs and implementation.

Acceptance criteria

• README distinguishes single-item security-event fields from batch span fields.
• The batch field list includes checker-level fields and nested per-policy span fields.
• Existing tracing contract tests still pass.