feat: load defaults into Config and introduce ConfigOverrides (#677)

This changes how instantiating `Config` works and also adds
`approval_policy` and `sandbox_policy` as fields. The idea is:

* All fields of `Config` have appropriate default values.
* `Config` is initially loaded from `~/.codex/config.toml`, so values in
`config.toml` will override those defaults.
* Clients must instantiate `Config` via
`Config::load_with_overrides(ConfigOverrides)` where `ConfigOverrides`
has optional overrides that are expected to be settable based on CLI
flags.

The `Config` should be defined early in the program and then passed
down. Now functions like `init_codex()` take fewer individual parameters
because they can just take a `Config`.

Also, `Config::load()` used to fail silently if `~/.codex/config.toml`
had a parse error and fell back to the default config. This seemed
really bad because it wasn't clear why the values in my `config.toml`
weren't getting picked up. I changed things so that
`load_with_overrides()` returns `Result<Config>` and verified that the
various CLIs print a reasonable error if `config.toml` is malformed.

Finally, I also updated the TUI to show which **sandbox** value is being
used, as we do for other key values like **model** and **approval**.
This was also a reminder that the various values of `--sandbox` are
honored on Linux but not macOS today, so I added some TODOs about fixing
that.

codex-rs/core/src/protocol.rs

@@ -26,7 +26,7 @@ pub enum Op {
     /// Configure the model session.
     ConfigureSession {
         /// If not specified, server will use its default model.
-        model: Option<String>,
+        model: String,
         /// Model instructions
         instructions: Option<String>,
         /// When to escalate for approval for execution
@@ -66,11 +66,13 @@ pub enum Op {
 }
 
 /// Determines how liberally commands are auto‑approved by the system.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "kebab-case")]
 pub enum AskForApproval {
     /// Under this policy, only “known safe” commands—as determined by
     /// `is_safe_command()`—that **only read files** are auto‑approved.
     /// Everything else will ask the user to approve.
+    #[default]
     UnlessAllowListed,
 
     /// In addition to everything allowed by **`Suggest`**, commands that
@@ -91,13 +93,15 @@ pub enum AskForApproval {
 }
 
 /// Determines execution restrictions for model shell commands
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "kebab-case")]
 pub enum SandboxPolicy {
     /// Network syscalls will be blocked
     NetworkRestricted,
     /// Filesystem writes will be restricted
     FileWriteRestricted,
     /// Network and filesystem writes will be restricted
+    #[default]
     NetworkAndFileWriteRestricted,
     /// No restrictions; full "unsandboxed" mode
     DangerousNoRestrictions,