Context

Phase 1 of #1741 removed anyhow from vault.rs, config/mod.rs, bootstrap/mod.rs, bootstrap/provider.rs, and daemon.rs.

However, anyhow remains as a dependency in crates/zeph-core/Cargo.toml because the following files still use it:

• crates/zeph-core/src/subagent/manager.rs
• crates/zeph-core/src/subagent/error.rs
• crates/zeph-core/src/config_watcher.rs

Acceptance Criteria

• Replace anyhow usages in subagent/manager.rs with typed SubAgentError (thiserror)
• Replace anyhow usages in subagent/error.rs with typed error variants
• Replace anyhow usages in config_watcher.rs with typed ConfigWatcherError (thiserror)
• Remove anyhow from crates/zeph-core/Cargo.toml
• All CI checks pass

Closes

Completes #1732.