feat: session and tree browsers

[dnouri]

Browse sessions and conversation trees in dedicated Emacs buffers,
replacing the old completing-read session picker. Both buffers stay
open alongside chat — no modal overlays.

Requires the dnouri/rpc-browsing-surface branch in pi-mono (not yet
on main). Adds magit-section as a package dependency.

Session browser (C-c C-r):
Find and switch sessions. Sort modes: threaded (shows fork
lineage with tree connectors), recent, relevance. Filter to
named-only, toggle current/all project scope, search with /.
Rename the active session with r (upstream only supports renaming
the current session). Right margin shows message count and age.

Tree browser (C-c C-p then w):
See the full branch structure of the current conversation. The
active path is highlighted; abandoned branches are dimmed. Five
filter modes cycle with f: no-tools (default), default, user-only,
labeled-only, all. RET navigates without summarizing; S prompts
for summary instructions then navigates with summarization.
C-c C-k aborts an in-flight summarization (sends
abort_branch_summary to the server). Label nodes with l,
search with /. Right margin shows labels.

Both buffers use magit-section for point-centric navigation and
action dispatch. ?/h opens a transient help menu in each buffer.
g refreshes from the server. q buries the buffer.

Other changes:

• render.el: display branchSummary messages in session history
  (separator + full markdown, same pattern as compactionSummary);
  scroll all chat windows to end after history reload.
• menu.el: remove resume-session and its disk-based helpers
  (session-dir-name, list-sessions, format-session-choice).
• core.el: normalize-string-or-null now treats empty strings as nil.
• ui.el: C-c C-r rebound to session-browser; startup header updated.
• Tests moved from menu/input to browse-test.el where they belong.
  119 browse-specific tests; 779 total, all green.

[dnouri]

Depends on badlogic/pi-mono#1628