session: stop truncating hashes in REPL/render output

Four sites in the session paths were truncating session_root and
cache_key to 12-16 hex chars + "…". These are cryptographic content
addresses — the WHOLE hash IS the identifier; truncation makes them
unusable for copy-paste and forensic reference.

Affected:
- session.py render_tree() header
- cli.py session --list output
- cli.py session REPL banner
- cli.py session REPL per-turn footer

Other hash truncations elsewhere in cli.py (search/inspect/burn paths)
are left alone — those are scan-and-skim views where ellipsis is the
intended affordance. Touch them only when they bite.

arborist/cli.py

@@ -7559,7 +7559,7 @@ def _cmd_session(args: argparse.Namespace) -> int:
             for s in listing:
                 print(
                     f"  {s['sid']:32s}  nodes={s['n_nodes']:3d}  "
-                    f"root={s['session_root'][:12]}…  "
+                    f"root={s['session_root']}  "
                     f"updated={s['updated_at']}"
                 )
         return 0
@@ -7647,7 +7647,7 @@ def _cmd_session(args: argparse.Namespace) -> int:
         sess = Session.open(p, sid=sid)
     print(f"# session {sess.sid}", file=sys.stderr)
     print(
-        f"# db {p}  ·  root {sess.session_root[:12]}…",
+        f"# db {p}  ·  root {sess.session_root}",
         file=sys.stderr,
     )
     print(
@@ -7853,8 +7853,8 @@ def _session_repl(args: argparse.Namespace, sess) -> int:
             print(f"\n[{n.bates}] {audit_mode}")
             print(ans)
             print(
-                f"\n# root → {sess.session_root[:16]}…  "
-                f"cache_key={ckey[:12]}…"
+                f"\n# root       {sess.session_root}"
+                f"\n# cache_key  {ckey}"
             )
     finally:
         try:

arborist/qa/session.py

@@ -577,7 +577,7 @@ def render_tree(sess: Session) -> str:
     bps = sess.branches()
     lines: list[str] = [
         f"session {sid} ({n_nodes} nodes · {len(bps)} branch points · "
-        f"root={sess.session_root[:12]}… · current → {cur})",
+        f"root={sess.session_root} · current → {cur})",
     ]
     root = sess.root_bates