desktop: cargo test --release fails to compile (storage_tests.rs uses debug_assertions-gated items without cfg gates)

## Problem

`cargo test --release` in `desktop/src-tauri` fails to compile with 11 E0425 errors. `src/managed_agents/storage_tests.rs` references `copy_agent_keys_between_stores` (storage.rs:515) and `DEV_MIGRATION_MARKER` (storage.rs:494), both `#[cfg(debug_assertions)]`, but the referencing tests carry no matching cfg gate (the only cfgs in storage_tests.rs are `#[cfg(unix)]` at :325 and :708).

Debug-profile `cargo test` passes (2430/0), which is why CI never sees it. Found while independently verifying PR #5763 at `37e056ed037f77b76e614c4b342bc8c6399e079b`; pre-existing — storage_tests.rs is untouched by that PR (last touched by d40a33290).

## Repro

```
cd desktop/src-tauri && cargo test --release
# error[E0425]: cannot find function `copy_agent_keys_between_stores` in module `super`
```

## Fix shape

Gate the migration tests (or the whole block of them) with `#[cfg(debug_assertions)]` to match the items under test.

Start in desktop/src-tauri/src/managed_agents/storage_tests.rs and compare its migration tests with the #[cfg(debug_assertions)] items in storage.rs at lines 494 and 515. Run cargo test --release from desktop/src-tauri to reproduce the E0425 errors, then verify the release test command compiles and passes after the tests are consistently gated.

Written by the indexing model from the issue text.