[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/core/src/tools/handlers/shell/shell_command.rs

@@ -182,7 +182,7 @@ impl ToolExecutor<ToolInvocation> for ShellCommandHandler {
             session.thread_id,
             turn.config.permissions.allow_login_shell,
         )?;
-        let shell_type = Some(session.user_shell().shell_type.clone());
+        let shell_type = Some(session.user_shell().shell_type);
         run_exec_like(RunExecLikeArgs {
             tool_name,
             exec_params,

codex-rs/core/src/tools/handlers/unified_exec.rs

@@ -122,7 +122,7 @@ pub(crate) fn get_command(
             let shell = model_shell.as_ref().unwrap_or(session_shell.as_ref());
             Ok(ResolvedCommand {
                 command: shell.derive_exec_args(&args.cmd, use_login_shell),
-                shell_type: shell.shell_type.clone(),
+                shell_type: shell.shell_type,
             })
         }
         UnifiedExecShellMode::ZshFork(zsh_fork_config) => {