## Why
`debug_clear_memories_resets_state_and_removes_memory_dir` can be flaky
because the test drops its `sqlx::SqlitePool` immediately before
invoking `codex debug clear-memories`. Dropping the pool does not wait
for all SQLite connections to close, so the CLI can race with still-open
test connections.
## What changed
- Await `pool.close()` before spawning `codex debug clear-memories`.
- Close the reopened verification pool before the temp `CODEX_HOME` is
torn down.
## Verification
- `cargo test -p codex-cli --test debug_clear_memories
debug_clear_memories_resets_state_and_removes_memory_dir`
#### what
adds a `codex debug clear-memories` command to help with clearing all
memories state from disk, sqlite db, and marking threads as
`memory_mode=disabled` so they don't get resummarized when the
`memories` feature is re-enabled.
#### tests
add tests
