core: remove cross-crate re-exports from lib.rs

[bolinfest]

Why

codex-core was re-exporting APIs owned by sibling codex-* crates, which made downstream crates depend on codex-core as a proxy module instead of the actual owner crate.

Removing those forwards makes crate boundaries explicit and lets leaf crates drop unnecessary codex-core dependencies. In this PR, this reduces the dependency on codex-core to codex-login in the following files:

codex-rs/backend-client/Cargo.toml
codex-rs/mcp-server/tests/common/Cargo.toml

What

• Remove codex-rs/core/src/lib.rs re-exports for symbols owned by codex-login, codex-mcp, codex-rollout, codex-analytics, codex-protocol, codex-shell-command, codex-sandboxing, codex-tools, and codex-utils-path.
• Delete the default_client forwarding shim in codex-rs/core.
• Update in-crate and downstream callsites to import directly from the owning codex-* crate.
• Add direct Cargo dependencies where callsites now target the owner crate, and remove codex-core from codex-rs/backend-client.