Add tmux-fzf for session/window/pane navigation

Many concurrent sessions with many panes each make it hard to track what is
running where, and moving a pane between windows means join-pane with a
hand-typed target, so it rarely gets done.

tmux-fzf gives a fuzzy-searchable list of every session, window and pane with
a live preview of each, and implements break/join/swap as pane actions --
which is the pane-movement gap. fzf is already in apt.global, arch.global and
brew.global, so no new packages are needed.

Checked out as a submodule rather than left for TPM to clone on first
prefix+I: ~/.tmux symlinks to config/tmux, so a TPM-installed plugin lands
inside this repo as untracked files. The submodule pins a version and
installs via the existing "git submodule update --init". TPM still sources it
from the @plugin line and can still update it.

prefix+F is the plugin's own top-level menu. prefix+P/W/S skip straight to
the pane, window and session panels. TMUX_FZF_CLIENT pins each action to the
client that opened it, as the plugin's own binding does -- run-shell has no
client context, so tmux otherwise targets the most recently active client,
which is wrong when a second client is attached to the same session.

No env-var configuration was needed: preview is already on by default, and
the TMUX_FZF_* variables are read from the environment only, so the README's
suggestion to set them in .tmux.conf is not valid tmux syntax.

Tested against tmux 3.5a and fzf 0.38.0: config parses with no errors, all
three bindings register, existing bindings are unaffected, TPM sources the
plugin and binds prefix+F, the three scripts pass bash -n, and the plugin's
fzf-version detection resolves to fzf-tmux with preview enabled. The
interactive picker itself needs a real terminal and is left to manual check.

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "dependencies/fzf"]
 	path = dependencies/fzf
 	url = https://github.com/junegunn/fzf.git
+[submodule "config/tmux/plugins/tmux-fzf"]
+	path = config/tmux/plugins/tmux-fzf
+	url = https://github.com/sainnhe/tmux-fzf

config/tmux.conf

@@ -47,6 +47,21 @@ bind -r C-l select-window -t :+
 
 bind a setw synchronize-panes
 
+# Fuzzy pickers over every session/window/pane, each with a live preview of
+# what it's running (tmux-fzf). prefix+F opens the plugin's own top-level
+# menu; these skip straight to the panels used most -- pane.sh is where
+# break/join/swap live, i.e. moving a pane into another window without
+# hand-typing a target the way join-pane needs.
+#
+# TMUX_FZF_CLIENT pins the action to the client that opened it, exactly as
+# the plugin's own binding does: a command run via run-shell has no client
+# context of its own, so tmux otherwise falls back to the most recently
+# active client -- the wrong terminal whenever a second client (e.g. a
+# browser one over ttyd) is attached to the same session.
+bind P run-shell -b "TMUX_FZF_CLIENT='#{client_tty}' ~/.tmux/plugins/tmux-fzf/scripts/pane.sh"
+bind W run-shell -b "TMUX_FZF_CLIENT='#{client_tty}' ~/.tmux/plugins/tmux-fzf/scripts/window.sh"
+bind S run-shell -b "TMUX_FZF_CLIENT='#{client_tty}' ~/.tmux/plugins/tmux-fzf/scripts/session.sh"
+
 # ----------------------------------------------------------------------------
 # Copy mode and mouse
 # ----------------------------------------------------------------------------
@@ -155,6 +170,13 @@ set -g status-style "#{?client_prefix,#{@status_style_armed},#{@status_style_nor
 # ----------------------------------------------------------------------------
 set -g @plugin 'tmux-plugins/tpm'
 set -g @plugin 'tmux-plugins/tmux-sensible'
+# Checked out as a submodule under config/tmux/plugins rather than left for
+# TPM to clone on first prefix+I: ~/.tmux is a symlink to that directory, so
+# a TPM-installed plugin lands inside this repo as untracked files. The
+# submodule pins a version, installs with the existing "git submodule
+# update --init" in install.sh, and keeps git status clean. TPM still sources
+# it from the @plugin line below and can still update it.
+set -g @plugin 'sainnhe/tmux-fzf'
 
 # Local overrides
 if-shell "[ -f ~/.tmux_local.conf ]" 'source ~/.tmux_local.conf'