[codex] Add environment shell info (#26480)

## Why

Shell detection needs to be available through the `Environment`
abstraction so callers can ask the selected local or remote environment
for shell metadata without adding a separate HTTP endpoint or parallel
info-source path. This keeps shell metadata shaped like the existing
environment-owned filesystem capability and lets remote environments
answer through exec-server JSON-RPC.

## What changed

- Added `environment/info` to the exec-server protocol/client/server and
exposed `Environment::info()`.
- Added local and remote environment info providers on `Environment`,
following the existing capability-provider pattern used for filesystem
access.
- Moved the shared shell detection logic into `codex-shell-command` and
kept core shell APIs as wrappers around that implementation.
- Returned shell metadata as `EnvironmentInfo { shell: ShellInfo }`
using the existing shell detection path.
- Added a remote environment test that calls `Environment::info()`
through an exec-server-backed environment.

## Validation

- `git diff --check`
- `just test -p codex-shell-command`
- `just test -p codex-core -E 'test(/shell::tests::/)'`\n- `just test -p
codex-exec-server environment`

codex-rs/exec-server/src/protocol.rs

@@ -19,6 +19,7 @@ pub const EXEC_TERMINATE_METHOD: &str = "process/terminate";
 pub const EXEC_OUTPUT_DELTA_METHOD: &str = "process/output";
 pub const EXEC_EXITED_METHOD: &str = "process/exited";
 pub const EXEC_CLOSED_METHOD: &str = "process/closed";
+pub const ENVIRONMENT_INFO_METHOD: &str = "environment/info";
 pub const FS_READ_FILE_METHOD: &str = "fs/readFile";
 pub const FS_WRITE_FILE_METHOD: &str = "fs/writeFile";
 pub const FS_CREATE_DIRECTORY_METHOD: &str = "fs/createDirectory";
@@ -64,6 +65,23 @@ pub struct InitializeResponse {
     pub session_id: String,
 }
 
+/// Information about an execution/filesystem environment.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct EnvironmentInfo {
+    pub shell: ShellInfo,
+}
+
+/// Shell detected for an execution/filesystem environment.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ShellInfo {
+    /// Stable shell name, for example `zsh`, `bash`, `powershell`, `sh`, or `cmd`.
+    pub name: String,
+    /// Path the exec server would use for that shell.
+    pub path: String,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ExecParams {