feat(c5): add zellij exit-capture logging and shared prune helper

Zellij has no live-stream facility like tmux pipe-pane, so capture is a
one-shot 'zellij action dump-screen --full' run from a fish_exit handler
(conf.d/zellij-logging.fish -> functions/_zellij_dump_log.fish). The C5
guard is checked inside the helper at exit time, so toggling logging
needs no sync_logging coordination — there is no persistent stream.

Extract pruning into functions/_prune_terminal_logs.fish, shared by both
the tmux and zellij helpers. This also fixes a latent bug in the tmux
helper: globbing tmux_*.log directly errored with 'No matches for
wildcard' on the first log in an empty dir. The shared helper globs via
'set' (tolerates no-match) and uses command ls/rm to bypass the eza and
trash C1 shadows.

functions/_zellij_dump_log.fish

@@ -0,0 +1,40 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _zellij_dump_log
+#
+# DESCRIPTION
+#   Captures the current Zellij pane's scrollback to a timestamped log in
+#   SCROLLBACK_HISTORY_DIR (default ~/.terminal_history). Zellij has no live
+#   output-streaming facility like tmux's pipe-pane, so this performs a one-shot
+#   `zellij action dump-screen --full` — intended to run on shell exit. Old
+#   zellij_*.log files are pruned via _prune_terminal_logs to stay within
+#   SCROLLBACK_HISTORY_MAX_FILES.
+#
+#   The C5 logging guard (__fish_config_op_logging) is evaluated here, at call
+#   time, so toggling logging takes effect on the next exit without a restart.
+#   No-op when logging is disabled, not inside Zellij ($ZELLIJ unset), or the
+#   zellij binary is missing.
+#
+# RETURNS
+#   0  Always
+#
+# EXAMPLE
+#   _zellij_dump_log
+function _zellij_dump_log --description 'Dump the current Zellij pane scrollback to a log file, with pruning'
+    __fish_config_op_enabled __fish_config_op_logging; or return 0
+    set -q ZELLIJ; or return 0
+    type -q zellij; or return 0
+
+    set -l log_dir (set -q SCROLLBACK_HISTORY_DIR; and echo $SCROLLBACK_HISTORY_DIR; or echo "$HOME/.terminal_history")
+    set -l session (set -q ZELLIJ_SESSION_NAME; and echo $ZELLIJ_SESSION_NAME; or echo unknown)
+    set -l pane_id (set -q ZELLIJ_PANE_ID; and echo $ZELLIJ_PANE_ID; or echo unknown)
+    set -l timestamp (date "+%Y-%m-%d_%H-%M-%S")
+    set -l log_file "$log_dir/zellij_"$session"-p"$pane_id"_"$timestamp".log"
+
+    mkdir -p $log_dir
+    zellij action dump-screen --full "$log_file" 2>/dev/null
+
+    _prune_terminal_logs zellij
+end