fix(fs): flush RealFs append to prevent data loss race (#586)

## Summary
- Adds `file.flush().await?` after `write_all` in `RealFs::append` to
ensure data is persisted before the file handle is dropped
- Fixes flaky `readwrite_mount_modifies_host` test that fails in CI due
to tokio's internal buffering — `write_all` returns `Ready` before the
blocking write completes, so without flush the data can be lost on drop

## Test plan
- [x] `cargo test --features realfs -p bashkit --test realfs_tests
readwrite_mount_modifies_host` passes
- [x] `cargo test --all-features --tests` passes
- [x] `cargo clippy --all-targets --all-features -- -D warnings` clean
- [ ] CI green

Author: chaliy

crates/bashkit/src/fs/realfs.rs

@@ -293,6 +293,7 @@ impl FsBackend for RealFs {
             .open(&real)
             .await?;
         file.write_all(content).await?;
+        file.flush().await?;
         Ok(())
     }