feat: add memory usage event metrics

crates/memory/src/workspace.rs

@@ -33,6 +33,8 @@ const SUMMARY_FILE: &str = "summary.md";
 const DECISIONS_DIR: &str = "decisions";
 const REQUESTS_DIR: &str = "requests";
 const STAGING_DIR: &str = "_staging";
+const USAGE_DIR: &str = "_usage";
+const USAGE_EVENTS_FILE: &str = "events.jsonl";
 
 /// What kind of record a path under the memory tree represents.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -126,6 +128,14 @@ impl WorkspaceLayout {
         self.memory_dir().join(STAGING_DIR)
     }
 
+    pub fn usage_dir(&self) -> PathBuf {
+        self.memory_dir().join(USAGE_DIR)
+    }
+
+    pub fn usage_events_path(&self) -> PathBuf {
+        self.usage_dir().join(USAGE_EVENTS_FILE)
+    }
+
     pub fn decision_path(&self, slug: &Slug) -> PathBuf {
         self.decisions_dir().join(format!("{slug}.md"))
     }
@@ -145,7 +155,7 @@ impl WorkspaceLayout {
     /// Classify a path under the memory tree. Returns `None` if the
     /// path is not under `.insomnia/memory/`, `.insomnia/knowledge/`,
     /// or `.insomnia/workflow/` of this workspace, or if it lives in
-    /// `_staging/` (which is opaque to the linter).
+    /// `_staging/` / `_usage/` (opaque subsystem-owned trees).
     ///
     /// On a conventional path that's *almost* a record but malformed
     /// (e.g. `.insomnia/memory/decisions/Foo.md` with an invalid slug),
@@ -182,8 +192,8 @@ impl WorkspaceLayout {
                 slug: None,
             }));
         }
-        if first == STAGING_DIR {
-            // Linter opts out of `_staging/`; extract handles its schema.
+        if first == STAGING_DIR || first == USAGE_DIR {
+            // Linter opts out of subsystem-owned opaque trees.
             return Ok(None);
         }
 
@@ -295,6 +305,14 @@ mod tests {
         );
     }
 
+    #[test]
+    fn usage_tree_is_opaque_to_classifier() {
+        let cp = layout()
+            .classify(&PathBuf::from("/ws/.insomnia/memory/_usage/events.jsonl"))
+            .unwrap();
+        assert!(cp.is_none());
+    }
+
     #[test]
     fn outside_returns_none() {
         assert!(