refactor: separate core plugins and desktop shell

[IchenDEV]

Summary

• Extract plugin composition, runtime, and bundle protocol into codetwo-plugins, leaving product invariants in codetwo-core and enforcing the boundary with architecture tests.
• Add a Desktop container boundary and split Settings and Dock content into focused modules while preserving session import, browser access, diagnostics export, responsive Dock behavior, and tiled session panes.
• Align shared window chrome to a compact 40 px titlebar baseline.

Validation

• bun test — 724 passed, 0 failed
• bun run build:renderer
• cargo check --workspace --all-targets (Xcode SDK compatibility shim)
• cargo test --workspace --quiet (Xcode SDK compatibility shim)
• git diff --check