[tests] Investigate opencode_discovery test pollution in full-suite run

[Zeus-Deus]

Context

Discovered while preparing PR #15 for merge: `agent_provider::opencode::discovery::tests::check_against_live_binary_when_present` passes in isolation but fails in the full `cargo test` suite on the `workspace-cloud-push` branch.

This is a separate test-pollution issue from the `process_kill` one that was fixed in commit `38e78d7` (which was caused by `std::env::set_var("HOME", ...)` in expand_tilde tests leaking globally). The opencode_discovery test was failing on this branch even AFTER that fix, and the test file itself was last touched in commit `0dcf05b` (pre-dating PR #15 entirely).

This means some new test added on `workspace-cloud-push` is polluting the opencode_discovery test, but it's not the expand_tilde set_var one (already fixed).

What's needed

1. Reproduce the failure:
   ```bash
   cargo test --manifest-path src-tauri/Cargo.toml --no-default-features --lib 2>&1 | grep -E "test result|opencode_discovery"
   ```
   On the merged-to-main branch this should show 1 failure (`agent_browser` — pre-existing) plus the opencode_discovery one.

2. Bisect to find the polluter:

   • Run with progressively narrower test selections until the opencode_discovery failure goes away
   • The polluter is in whatever test was last excluded

3. Identify the pollution mechanism:

   • Likely candidates: a test that mutates env vars (similar to the expand_tilde bug), opens fds without closing them, spawns a process and leaks the pid, etc.
   • The opencode_discovery test probably does file/binary detection, so any pollution that affects PATH, file existence checks, or process spawning could trip it

4. Fix it:

   • Refactor the polluting test to not touch global state, OR
   • Save/restore the global state explicitly, OR
   • Mark the test as `serial_test` if reliable isolation is impossible

Acceptance criteria

`cargo test --manifest-path src-tauri/Cargo.toml --no-default-features --lib` runs with no failures except the pre-existing `agent_browser::resolve_binary_finds_native_binary_from_project_root` flake (which exists on main and is unrelated).

Out of scope

• Fixing the agent_browser flake (separate pre-existing issue)
• Rewriting the test infrastructure to avoid all pollution categorically

Estimate

30 min to 2 hours depending on how quickly the bisection narrows.