Replace CPEX core with facade crate

[lucarlig]

Summary

• replace direct git cpex-core dependency with crates.io cpex 0.2 facade
• update gateway CPEX imports to use cpex::cpex_core re-exported modules
• bump workspace rust-version to 1.96 to match cpex 0.2 MSRV
• update demo plugin git rev to merged contextforge-gateway-rs/cpex-plugins-rs main commit ab47801daccfbba44ea07b033034a347e7b5afdd

Verification

• cargo check --workspace --all-targets --all-features
• cargo test -p contextforge-gateway-rs-cpex
• cargo test -p contextforge-gateway-rs-lib --test gateway_plugins
• confirmed no cpex-sdk, contextforge-plugins-framework, or cpex-core 0.1.0 refs remain