feat(bench): add zeph-bench crate scaffold and BenchmarkChannel

[bug-ops]

Description

Create the crates/zeph-bench/ workspace crate with the bench feature flag and implement BenchmarkChannel that satisfies zeph-core::Channel.

Part of epic #2827. See spec: .local/specs/zeph-bench/spec.md.

Scope

• crates/zeph-bench/Cargo.toml with dependency on zeph-core, zeph-memory, zeph-llm, zeph-config
• BenchmarkChannel struct implementing all Channel trait methods (headless: recv from scenario queue, send/send_chunk collect to buffer, confirm=auto-approve, elicit=auto-decline, supports_exit=false, all others no-op)
• BenchError typed error enum via thiserror
• Unit tests for channel behavior (recv drains queue, send accumulates response, recv returns None when queue empty)
• Add bench feature to root Cargo.toml [features] (not included in full)
• Add dep:zeph-bench and register in workspace dependencies

Acceptance Criteria

• cargo build --features bench compiles with zero warnings
• BenchmarkChannel::recv returns turns from an injected queue and None when exhausted
• BenchmarkChannel::send and send_chunk accumulate response into an internal buffer readable by the harness
• BenchmarkChannel::confirm returns Ok(true) without blocking
• BenchmarkChannel::elicit returns Ok(Declined) without blocking
• Unit tests pass
• bench feature absent from full bundle