tmux: add pressure-based activity tracker

Add heartbeat.sh that tracks session active time using three native
tmux signals: client input (+5), window output (+2), and copy mode
(+2), with -1 decay per tick. Score drives a block bar indicator
and accumulated active time counter. Resets on tmux server restart.

Status bar updates:
- Heartbeat pill with pressure bar right of clock
- Responsive breakpoints: hide network/battery <120 cols, date <80
- Click clock/date to toggle condensed view
- Click heartbeat to instant-lock session
- Static colon in clock (replaces shell-based blinker)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

home/.tmux/heartbeat.sh

@@ -0,0 +1,114 @@
+#!/usr/bin/env bash
+# Heartbeat / liveness indicator for tmux status bar.
+#
+# Pressure-based activity tracking using native tmux signals:
+#   Input (keystroke/mouse):  +5 per event
+#   Output (command output):  +2 per tick
+#   Copy mode (reading):      +2 per tick
+#   Decay:                    -1 per tick
+#   Score capped at 100, floored at 0.
+#
+# Counter ticks while score > 0. Resets on tmux restart.
+#
+# Visual (pill with pressure bar):
+#   23m ▆  — teal time + teal bar (active, score > 0)
+#   5m     — gray time, no bar (paused, score = 0)
+#   10s ▃  — amber time + amber bar (returning from away)
+
+SCORE_CAP=100
+
+state="${TMPDIR:-/tmp}/.tmux_heartbeat"
+read client_act win_act in_mode tmux_pid <<< \
+    "$(tmux display-message -p '#{client_activity} #{window_activity} #{pane_in_mode} #{pid}')"
+now=$(date +%s)
+
+# Load previous state
+prev_client="" prev_win="" score=0 active_total=0 srv_pid=""
+return_at="" return_show=0
+[[ -f "$state" ]] && source "$state"
+
+# Reset if tmux server restarted
+if [[ "$srv_pid" != "$tmux_pid" ]]; then
+    score=0 active_total=0 return_at="" return_show=0
+fi
+
+# Detect screensaver/lock gaps (script wasn't running for >10s)
+if [[ -f "$state" ]]; then
+    last_run=$(stat -f%m "$state" 2>/dev/null || stat -c%Y "$state" 2>/dev/null || echo "$now")
+    gap=$(( now - last_run ))
+    if (( gap > 10 )); then
+        return_at=$now
+        return_show=$gap
+        score=0
+    fi
+fi
+
+# Score: check activity signals
+gaining=0
+
+if [[ -n "$prev_client" && "$client_act" != "$prev_client" ]]; then
+    (( score += 5 )); gaining=1
+fi
+
+if [[ -n "$prev_win" && "$win_act" != "$prev_win" ]]; then
+    (( score += 2 )); gaining=1
+fi
+
+if [[ "$in_mode" == "1" ]]; then
+    (( score += 2 )); gaining=1
+fi
+
+(( score-- ))
+(( score > SCORE_CAP )) && score=$SCORE_CAP
+(( score < 0 )) && score=0
+
+(( score > 0 )) && (( active_total++ ))
+
+# Clear return flash after 4s
+if [[ -n "$return_at" ]] && (( now - return_at >= 4 )); then
+    return_at="" return_show=0
+fi
+
+# Persist
+printf 'prev_client=%s\nprev_win=%s\nscore=%s\nactive_total=%s\nsrv_pid=%s\nreturn_at=%s\nreturn_show=%s\n' \
+    "$client_act" "$win_act" "$score" "$active_total" "$tmux_pid" "$return_at" "$return_show" > "$state"
+
+# Format seconds to ≤3 chars
+fmt() {
+    local s=$1
+    if (( s < 60 )); then printf "%ds" "$s"
+    elif (( s < 3600 )); then printf "%dm" $(( s / 60 ))
+    elif (( s < 86400 )); then printf "%dh" $(( s / 3600 ))
+    else printf "%dd" $(( s / 86400 )); fi
+}
+
+# Pressure bar: block fill (8 levels)
+bar() {
+    local s=$1
+    if (( s >= 88 )); then printf '█'
+    elif (( s >= 75 )); then printf '▇'
+    elif (( s >= 63 )); then printf '▆'
+    elif (( s >= 50 )); then printf '▅'
+    elif (( s >= 38 )); then printf '▄'
+    elif (( s >= 25 )); then printf '▃'
+    elif (( s >= 13 )); then printf '▂'
+    elif (( s >= 1 )); then printf '▁'
+    else printf ' '; fi
+}
+
+# Render: pill with margin (space + bg pill + space)
+render() {
+    local bar_fg=$1 b=$2 time_fg=$3 txt=$4 len=${#4}
+    local rpad=""
+    (( len < 3 )) && printf -v rpad "%$(( 3 - len ))s" ""
+    printf " #[bg=#1A1A1A,fg=%s] %s%s #[fg=%s]%s#[default] " "$time_fg" "$txt" "$rpad" "$bar_fg" "$b"
+}
+
+b=$(bar $score)
+if [[ -n "$return_at" ]]; then
+    render "colour208" "$b" "colour208" "$(fmt $return_show)"
+elif (( score > 0 )); then
+    render "#2CB494" "$b" "#2CB494" "$(fmt $active_total)"
+else
+    render "#808080" " " "#808080" "$(fmt $active_total)"
+fi