## Why
Some Linux environments expose `bash` at `/usr/bin/bash` instead of
`/bin/bash`. The shell detection fallback list should cover both
standard locations once PATH/user-shell probing fails.
Stacked on #26480.
## What changed
- Add `/usr/bin/bash` to the bash fallback path list in
`codex-shell-command`.
- Extend shell type detection coverage for `/usr/bin/bash`.
- Add AGENTS.md testing guidance to avoid tests for statically defined
values and negative tests for removed logic.
## Verification
- `just test -p codex-shell-command`
## 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`
