[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/shell-command/Cargo.toml

@@ -11,6 +11,7 @@ workspace = true
 base64 = { workspace = true }
 codex-protocol = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
+libc = { workspace = true }
 once_cell = { workspace = true }
 regex = { workspace = true }
 serde = { workspace = true, features = ["derive"] }