Two-pane session browser for /resume with AI-naming hook

Replace the single-list autosave picker with a PROJECTS|sessions
browser: sessions grouped by scope_key (legacy scope-less sessions in
an '(unscoped)' bucket pinned last), date-bucketed two-line rows with
lazy title derivation cached back into the sidecar, global search
reachable from anywhere, sort cycling, d-then-y delete, and a message
preview overlay. Every composed row obeys a fit contract (left
truncates, right chrome drops whole units, never exceeds terminal
width) with an invariant test at 60-140 cols.

New session_browser_open callback hook fires with the browser's live
metadata dicts so plugins (session_namer) can enrich titles/tags in
the background and surface them on repaint.

Author: mpfaffenberger

code_puppy/callbacks.py

@@ -71,6 +71,7 @@
     "pre_compact",
     "session_end",
     "post_autosave",
+    "session_browser_open",
     "notification",
     "awaiting_user_input",
     "git_branch_provider",
@@ -159,6 +160,7 @@ def __repr__(self) -> str:
     "pre_compact": [],
     "session_end": [],
     "post_autosave": [],
+    "session_browser_open": [],
     "notification": [],
     "awaiting_user_input": [],
     "git_branch_provider": [],
@@ -590,6 +592,18 @@ async def on_post_autosave(*args, **kwargs) -> List[Any]:
     return await _trigger_callbacks("post_autosave", *args, **kwargs)
 
 
+async def on_session_browser_open(*args, **kwargs) -> List[Any]:
+    """Fire when the ``/resume`` session browser is about to open.
+
+    Receives ``(base_dir: str, entries: list[tuple[str, dict]])`` where
+    each entry is ``(session_name, metadata_dict)``. The metadata dicts
+    are the browser's LIVE objects: plugins that enrich them in place
+    (titles, tags) surface on the browser's next repaint. Handlers must
+    return fast -- do slow work (model calls) on a background thread.
+    """
+    return await _trigger_callbacks("session_browser_open", *args, **kwargs)
+
+
 def on_load_prompt():
     """Collect load_prompt fragments from plugins, dropping ``None`` results.