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/_prune_terminal_logs.fish

@@ -0,0 +1,45 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _prune_terminal_logs <prefix>
+#
+# DESCRIPTION
+#   Removes the oldest "<prefix>_*.log" files in SCROLLBACK_HISTORY_DIR
+#   (default ~/.terminal_history) so the total count for that prefix stays
+#   within SCROLLBACK_HISTORY_MAX_FILES (default 100). Files are sorted by
+#   modification time, so the most recently written logs are kept — actively
+#   appended logs (e.g. a tmux pipe-pane stream) survive.
+#
+#   Uses `command ls`/`command rm` to bypass the C1 shadows: the bare `ls` is
+#   the eza wrapper, which injects OSC-8 hyperlink escapes into paths, and the
+#   bare `rm` is the trash wrapper. The glob is expanded via `set` first so a
+#   no-match (empty dir / first run) yields an empty list instead of a hard
+#   "No matches for wildcard" error.
+#
+# ARGUMENTS
+#   prefix  Log-name prefix to prune (e.g. tmux, zellij)
+#
+# RETURNS
+#   0  Always
+#
+# EXAMPLE
+#   _prune_terminal_logs tmux
+function _prune_terminal_logs --argument-names prefix --description 'Prune oldest <prefix>_*.log files beyond the configured max'
+    set -q prefix[1]; or return 0
+    set -l log_dir (set -q SCROLLBACK_HISTORY_DIR; and echo $SCROLLBACK_HISTORY_DIR; or echo "$HOME/.terminal_history")
+    set -l max_files (set -q SCROLLBACK_HISTORY_MAX_FILES; and echo $SCROLLBACK_HISTORY_MAX_FILES; or echo 100)
+    test -d $log_dir; or return 0
+
+    # Glob via `set` so a no-match yields an empty list rather than an error.
+    set -l logs $log_dir/$prefix"_"*.log
+    set -q logs[1]; or return 0
+
+    # Re-sort by mtime, oldest first. Pass explicit files (no glob) so ls can
+    # never fail on no-match; `command ls` avoids the eza OSC-8 corruption.
+    set logs (command ls -1tr $logs 2>/dev/null)
+    if test (count $logs) -gt $max_files
+        set -l excess (math (count $logs) - $max_files)
+        command rm -f $logs[1..$excess]
+    end
+end