feat(editor): add :enew and :delete with real git-staging (v0.5 slice 3)

:enew <name> creates a new file (empty, dirty, added to the palette list);
:delete unlinks the current file via a new Effect::Delete and switches to a
parked buffer or scratch. Scope is read from the path (local/x, repo/x) rather
than a modal prompt, and the /sd prefix is now optional in resolve_path.

On-device testing showed deletions never reached the remote: add_all(["*"])
alone does not stage a removal on this libgit2, so the tree came back unchanged
and the push was a silent no-op. stage_and_commit now runs add_all then
update_all(["*"]) (git add -u) — together git add -A. The :delete snackbar now
confirms the scoped file and flags that a Tracked file is local until :sync.

firmware/src/persistence.rs

@@ -308,6 +308,20 @@ impl Storage {
         Ok(())
     }
 
+    /// Unlink a file under `/sd` (`:delete`). Tolerates a missing target — an
+    /// already-gone file is a success, so the call is idempotent. Also clears a
+    /// stray `{path}.tmp` best-effort, so a crash-interrupted save can't leave the
+    /// file half-present after a delete. For a Tracked file this leaves the
+    /// working copy short one file; the next publish's `add --all` stages it.
+    pub fn delete_path(&self, path: &str) -> Result<()> {
+        let _ = fs::remove_file(format!("{path}.tmp"));
+        match fs::remove_file(path) {
+            Ok(()) => Ok(()),
+            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(()),
+            Err(e) => Err(e).with_context(|| format!("unlink {path}")),
+        }
+    }
+
     /// Reconcile a leftover `notes.md.tmp` at boot. The save sequence is
     /// write-tmp → fsync → unlink-target → rename, so a lingering tmp means the
     /// last save was interrupted. Which way to recover depends on whether the