Engine: anonymous usage events, counted in Session::execute (#238)

fieldcraft_engine::usage holds the events docs/privacy.md lists, typed so
that only registry names (command ids, sample names, requirements) and
durations can go in them, a bounded queue that records nothing until the
host enables it, and the PostHog batch body. Session::execute counts each
top-level command of a session with an origin (ui, palette, script, mcp):
its id, kind, duration and error class, and the funnel's milestones and
opened samples. Nothing here touches the network.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/file.rs

@@ -236,12 +236,13 @@ impl Session {
     }
 
     /// Replace this session with `opened`, keeping its undo memory limit, its work caps and its
-    /// file access; the history starts empty.
+    /// file access (and where its commands come from); the history starts empty.
     fn replace_with(&mut self, mut opened: Session) {
         opened.history = std::mem::take(&mut self.history);
         opened.limits = self.limits;
         opened.file_access = std::mem::take(&mut self.file_access);
         opened.crash_journal = self.crash_journal;
+        opened.origin = self.origin;
         opened.clear_history();
         opened.mark_opened();
         *self = opened;