core: remove cross-crate re-exports from lib.rs (#16512)

## 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`.

codex-rs/core/src/exec_policy.rs

@@ -7,8 +7,6 @@ use arc_swap::ArcSwap;
 
 use crate::config_loader::ConfigLayerStack;
 use crate::config_loader::ConfigLayerStackOrdering;
-use crate::is_dangerous_command::command_might_be_dangerous;
-use crate::is_safe_command::is_known_safe_command;
 use codex_execpolicy::AmendError;
 use codex_execpolicy::Decision;
 use codex_execpolicy::Error as ExecPolicyRuleError;
@@ -25,16 +23,18 @@ use codex_protocol::permissions::FileSystemSandboxKind;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_shell_command::is_dangerous_command::command_might_be_dangerous;
+use codex_shell_command::is_safe_command::is_known_safe_command;
 use thiserror::Error;
 use tokio::fs;
 use tokio::task::spawn_blocking;
 use tracing::instrument;
 
-use crate::bash::parse_shell_lc_plain_commands;
-use crate::bash::parse_shell_lc_single_command_prefix;
 use crate::config::Config;
 use crate::sandboxing::SandboxPermissions;
 use crate::tools::sandboxing::ExecApprovalRequirement;
+use codex_shell_command::bash::parse_shell_lc_plain_commands;
+use codex_shell_command::bash::parse_shell_lc_single_command_prefix;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use shlex::try_join as shlex_try_join;