feat(c5): add tmux pipe-pane session logging

Starts tmux pipe-pane capture for the current pane whenever fish
launches inside a tmux session. Each shell gets a timestamped log
in SCROLLBACK_HISTORY_DIR. Toggling __fish_config_op_logging stops
or restarts the pipe in all open fish shells via the existing
--on-variable event handlers.

conf.d/tmux-logging.fish

@@ -0,0 +1,19 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# C5 — Logging & Capture: starts a pipe-pane log for the current tmux pane
+# when fish launches inside a tmux session. Each fish shell gets its own
+# timestamped log file in SCROLLBACK_HISTORY_DIR (default: ~/.terminal_history).
+# Naming: tmux_<session>-w<window>-p<pane>_YYYY-MM-DD_HH-MM-SS.log
+
+__fish_config_op_enabled __fish_config_op_logging; or exit
+status is-interactive; or exit
+type -q tmux; or exit
+set -q TMUX; or exit
+
+set -l log_dir (set -q SCROLLBACK_HISTORY_DIR; and echo $SCROLLBACK_HISTORY_DIR; or echo "$HOME/.terminal_history")
+set -l pane_id (tmux display-message -p '#{session_name}-w#{window_index}-p#{pane_index}' 2>/dev/null)
+set -l timestamp (date "+%Y-%m-%d_%H-%M-%S")
+
+mkdir -p $log_dir
+tmux pipe-pane "cat >> $log_dir/tmux_${pane_id}_${timestamp}.log" 2>/dev/null