Plain POSIX files: remove commit on close (fs.rs)

Staging every write until close broke tempfile, git, and SQLite, which
expect normal files. Actors that need an atomic publish write a temporary
file and rename it; the SDK has write_atomic for that.

AGENTS.md

@@ -21,7 +21,7 @@ Keep it that way.
   - `lib.rs`: `Host`, which starts and stops a world.
   - `broker.rs`: routing, mailboxes, the version store, broker requests.
   - `supervisor.rs`: runs one WASM actor (restarts, swaps, rollbacks).
-  - `runtime.rs`: Wasmtime instances. `fs.rs`: commit on close.
+  - `runtime.rs`: Wasmtime instances and what they get (mounts, network).
   - `candidate.rs`: tests a new version. `cargo_actor.rs`: builds Rust.
   - `world.rs`: the on-disk layout. `console.rs`: the terminal.
 - `sdk/`: `sausage-actor`, the SDK for Rust actors. Never published: the