update paths for new agent tree architecture

- sessions-dir: s/ → session/
- Add ellie--agent-dir to discover active agent ID
- ellie--session-file routes agent files (chat, prompt, cfg, ctl, etc.)
  through session/{sid}/agent/{aid}/, session-level files (plan, env)
  stay at session/{sid}/.

ellie.el

@@ -90,18 +90,39 @@ An alist of (KEY . VALUE) string pairs; valid keys are
 ;;;; ──────────────── Paths ────────────────
 
 (defun ellie--sessions-dir ()
-  (expand-file-name "s" ellie-mount-directory))
+  (expand-file-name "session" ellie-mount-directory))
 
 (defun ellie--sessions-new-path ()
   (expand-file-name "new" (ellie--sessions-dir)))
 
+(defun ellie--agent-dir ()
+  "Return the path to the active agent directory for the current session.
+Discovers the agent ID by reading the agent/ subdirectory."
+  (unless ellie--session-id
+    (user-error "No active ollie session"))
+  (let* ((agent-parent (expand-file-name
+                        "agent"
+                        (expand-file-name ellie--session-id (ellie--sessions-dir))))
+         (entries (and (file-directory-p agent-parent)
+                       (directory-files agent-parent nil "\\`[^.]"))))
+    (expand-file-name
+     (or (seq-find (lambda (e)
+                     (file-directory-p (expand-file-name e agent-parent)))
+                   entries)
+         "0")
+     agent-parent)))
+
 (defun ellie--session-file (name)
-  "Return the full path to file NAME inside the current session directory.
+  "Return the full path to file NAME for the current session.
+Agent files (chat, prompt, cfg, ctl, etc.) resolve under the agent directory.
+Session-level files (plan, env) resolve under the session directory.
 Signals `user-error' if no session is active."
   (unless ellie--session-id
     (user-error "No active ollie session"))
-  (expand-file-name name
-                    (expand-file-name ellie--session-id (ellie--sessions-dir))))
+  (if (member name '("plan" "env"))
+      (expand-file-name name
+                        (expand-file-name ellie--session-id (ellie--sessions-dir)))
+    (expand-file-name name (ellie--agent-dir))))
 
 (defun ellie--last-session-path ()
   (expand-file-name "~/.config/ollie/last-session"))