tech-debt: remove #[allow(clippy::unused_async)] crate-root allow in zeph-agent-context

[bug-ops]

Description

crates/zeph-agent-context/src/lib.rs:4 has a #[allow(clippy::unused_async)] applied at the crate root with a comment "Allow async stubs during scaffold phase". This blanket allow suppresses warnings for all async functions in the crate, masking any future incomplete stubs or accidentally empty async methods.

The scaffold phase is over — all ContextService methods have been filled in. This allow should be removed so that Clippy can catch any future accidentally empty async functions added to this crate.

Reproduction Steps

1. Remove #[allow(clippy::unused_async)] from crates/zeph-agent-context/src/lib.rs
2. Run cargo clippy -p zeph-agent-context -- -D warnings
3. Observe: either zero warnings (allow was unnecessary) or specific stub methods that need real implementations

Expected Behavior

No #[allow(clippy::unused_async)] at crate root; lint is enabled so future accidental stubs are caught.

Actual Behavior

Blanket allow at crate level silently suppresses the lint for all public async fn in ContextService.

Environment

• Crate: zeph-agent-context
• File: crates/zeph-agent-context/src/lib.rs:4
• Detected: CI-892 arch audit

Logs / Evidence

// lib.rs line 4
#[allow(clippy::unused_async)]