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.

conf.d/zellij-logging.fish

@@ -0,0 +1,24 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# C5 — Logging & Capture: registers a fish_exit handler that dumps the current
+# Zellij pane's scrollback to a log file when the shell exits. Unlike tmux
+# (streamed live via pipe-pane), Zellij has no continuous-capture facility, so
+# we snapshot once on exit via `zellij action dump-screen --full`.
+#
+# The handler is registered whenever fish runs inside Zellij; the C5 logging
+# guard is evaluated inside _zellij_dump_log at exit time, so toggling
+# __fish_config_op_logging takes effect on the next exit without a restart
+# (no sync_logging coordination needed, since there is no live stream to stop).
+#
+# Event-handler functions must be defined at startup (conf.d) so the --on-event
+# binding is registered; autoloaded functions in functions/ never register.
+
+status is-interactive; or exit
+type -q zellij; or exit
+set -q ZELLIJ; or exit
+
+function __zellij_dump_on_exit --on-event fish_exit \
+    --description 'C5 event handler: dump Zellij pane scrollback on shell exit'
+    _zellij_dump_log
+end