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.

DECISIONS.md

@@ -39,8 +39,9 @@ actor, or a change that the agent makes to itself.
 
 ## Small choices
 
-- **Commit on close** for new and truncated files, so readers never see half
-  a file, in any language. Appends write in place, so logs stay cheap.
+- **Plain POSIX files.** An earlier version staged every write until close.
+  That broke `tempfile`, git, and SQLite, which expect normal files. Atomic
+  publishing is the usual temporary file + `rename`.
 - **Test first, then switch.** A new version is tested where it lies. Only
   a version that passes goes to `versions/`, and the old one runs until
   then. `versions/` keeps every version that was installed, so a rollback