[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/server/handler.rs

@@ -14,6 +14,7 @@ use tokio_util::task::TaskTracker;
 use crate::ExecServerRuntimePaths;
 use crate::client::http_client::PendingReqwestHttpBodyStream;
 use crate::client::http_client::ReqwestHttpRequestRunner;
+use crate::protocol::EnvironmentInfo;
 use crate::protocol::ExecParams;
 use crate::protocol::ExecResponse;
 use crate::protocol::FsCanonicalizeParams;
@@ -147,6 +148,11 @@ impl ExecServerHandler {
         session.process().exec(params).await
     }
 
+    pub(crate) fn environment_info(&self) -> Result<EnvironmentInfo, JSONRPCErrorError> {
+        self.require_initialized_for("environment info")?;
+        Ok(EnvironmentInfo::local())
+    }
+
     pub(crate) async fn exec_read(
         &self,
         params: ReadParams,