chore: remove unused async-trait dependency from 3 crates

[bug-ops]

Background

Plan item 01-06 proposed removing async-trait from zeph-acp, zeph-core, zeph-mcp.

Findings

Contrary to the initial analysis, async-trait IS actively used in all 3 crates:

• zeph-acp: #[async_trait::async_trait(?Send)] in terminal.rs, permission.rs, agent/mod.rs, fs.rs (~18 usages)
• zeph-core: #[async_trait::async_trait] in bootstrap/oauth.rs
• zeph-mcp: #[async_trait::async_trait] in client.rs

The ?Send variant in zeph-acp is particularly important — it handles non-Send types which cannot be migrated to native async traits without architectural changes.

Action required

This issue is deferred until Edition 2024 native async traits fully support ?Send semantics.
Track separately as a future improvement.