Fix env var state leakage in tests via Drop-based EnvGuard

[Copilot]

Tests mutating env vars (AGENT_SCOPES, UPDATE_SERVER_URL, UPDATE_STORAGE_PATH) used unconditional remove_var cleanup, leaking state into other tests on panic and clobbering any pre-existing env values. load_manifest_nonexistent_returns_default also used a hard-coded /tmp path that could become flaky if that path existed from a prior run.

Changes

• src/test_utils.rs (new) — shared EnvGuard drop-based restore helper; saves the original value before mutation and restores it on Drop regardless of panic:

let _env = EnvGuard::set("AGENT_SCOPES", "docker:restart,admin");
// original value restored automatically when _env drops, even on panic

• src/lib.rs — registers test_utils as #[cfg(test)] pub(crate) mod test_utils

• src/security/scopes.rs, src/commands/version_check.rs, src/commands/deploy.rs — replace all manual set_var/remove_var pairs with EnvGuard::set()/EnvGuard::remove() imported from the shared module

• src/commands/deploy.rs — load_manifest_nonexistent_returns_default now uses tempfile::tempdir() instead of /tmp/status-test-nonexistent-path for guaranteed isolation

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