feat: redesign sandbox config (#1373)

This is a major redesign of how sandbox configuration works and aims to
fix https://github.com/openai/codex/issues/1248. Specifically, it
replaces `sandbox_permissions` in `config.toml` (and the
`-s`/`--sandbox-permission` CLI flags) with a "table" with effectively
three variants:

```toml
# Safest option: full disk is read-only, but writes and network access are disallowed.
[sandbox]
mode = "read-only"

# The cwd of the Codex task is writable, as well as $TMPDIR on macOS.
# writable_roots can be used to specify additional writable folders.
[sandbox]
mode = "workspace-write"
writable_roots = []  # Optional, defaults to the empty list.
network_access = false  # Optional, defaults to false.

# Disable sandboxing: use at your own risk!!!
[sandbox]
mode = "danger-full-access"
```

This should make sandboxing easier to reason about. While we have
dropped support for `-s`, the way it works now is:

- no flags => `read-only`
- `--full-auto` => `workspace-write`
- currently, there is no way to specify `danger-full-access` via a CLI
flag, but we will revisit that as part of
https://github.com/openai/codex/issues/1254

Outstanding issue:

- As noted in the `TODO` on `SandboxPolicy::is_unrestricted()`, we are
still conflating sandbox preferences with approval preferences in that
case, which needs to be cleaned up.

codex-rs/exec/src/cli.rs

@@ -1,7 +1,6 @@
 use clap::Parser;
 use clap::ValueEnum;
 use codex_common::CliConfigOverrides;
-use codex_common::SandboxPermissionOption;
 use std::path::PathBuf;
 
 #[derive(Parser, Debug)]
@@ -23,9 +22,6 @@ pub struct Cli {
     #[arg(long = "full-auto", default_value_t = false)]
     pub full_auto: bool,
 
-    #[clap(flatten)]
-    pub sandbox: SandboxPermissionOption,
-
     /// Tell the agent to use the specified directory as its working root.
     #[clap(long = "cd", short = 'C', value_name = "DIR")]
     pub cwd: Option<PathBuf>,