feat: multi-platform alerting + shared system.env config with dynamic help values

- lib/notify.sh: route notify_send to every platform in NOTIFY_PLATFORM
  (notify.env, default telegram; comma-separated = send to all). New
  platforms need only a bin/pos-communication-<p> sender implementing
  'send <value> [--markdown]' (Matrix/Synapse ready)
- pos-communication-telegram: add --markdown as alias for --parse-mode
  markdown to match the sender contract
- lib/common.sh: load_system_env() — shared ~/.config/linux_post_install/
  system.env for pos-system-* tools (env exported > file > default)
- pos-system-health/backup: load system.env and show effective dynamic
  values (NOTIFY_PLATFORM, HEALTH_BACKUP_MAX_AGE_DAYS, BACKUP_SERVICE_ROOTS)
  in --help
- config/system.env + config/notify.env templates copied by postinstall
- systemd/pos-health.service: EnvironmentFile for both configs so the
  daily digest honors them

lib/common.sh

@@ -117,5 +117,24 @@ confirm() {
     fi
 }
 
+# ── system.env loader ──────────────────────────────────────────
+# Shared "system" tool config (~/.config/linux_post_install/system.env).
+# Fills only variables that are not already exported — an explicitly-set
+# environment variable always wins (flags > environment > file).
+load_system_env() {
+    local f="$HOME/.config/linux_post_install/system.env" k v
+    [ -f "$f" ] || return 0
+    while IFS='=' read -r k v; do
+        [ -n "$k" ] || continue
+        case "$k" in
+            \#*) continue ;;
+        esac
+        v="${v%\"}"; v="${v#\"}"; v="${v%\'}"; v="${v#\'}"
+        if [ -z "${!k:-}" ]; then
+            export "$k"="$v"
+        fi
+    done < <(grep -E '^[A-Z_]+=' "$f" || true)
+}
+
 # ── Source guard ───────────────────────────────────────────────
 return 0 2>/dev/null || true

lib/notify.sh

@@ -1,16 +1,39 @@
 #!/usr/bin/env bash
-# lib/notify.sh — optional alerting helper. Self-contained by design:
-# defines ONLY notify_send() so it can be sourced by tools that define
-# their own log/warn/err (e.g. pos-system-firewall) without clobbering.
+# lib/notify.sh — optional MULTI-PLATFORM alerting helper. Self-contained by
+# design: defines ONLY notify_send() + notify_platforms() (plus internal
+# helpers) so it can be sourced by tools that define their own log/warn/err
+# (e.g. pos-system-firewall) without clobbering.
 #
 # Usage (opt-in — source it, do NOT auto-load from common.sh):
 #   source "$(dirname "$0")/../lib/notify.sh" 2>/dev/null || source "$(dirname "$0")/notify.sh"
 #   notify_send "Backup completed: $ARCHIVE"
-#   notify_send "⚠ disk full" --markdown
+#   notify_send "**disk full**" --markdown
 #
-# Delegates to `pos communication telegram send`; silent-fail if the
-# telegram sender is missing or not configured (warns, never breaks the
-# caller and never changes its exit code).
+# Platform routing — ~/.config/linux_post_install/notify.env:
+#   NOTIFY_PLATFORM=telegram,matrix
+# Comma-separated = send to every listed platform (default: telegram).
+#
+# Sender contract — each platform is a bin/pos-communication-<platform> tool
+# that MUST implement:
+#   pos-communication-<platform> send <value> [--markdown]
+#   (exit 0 on delivery; non-zero on failure)
+# To add a platform (e.g. Matrix/Synapse), add `bin/pos-communication-matrix`
+# implementing that interface and put `matrix` in NOTIFY_PLATFORM.
+#
+# Silent-fails per platform: a missing sender or a failed send only warns and
+# never changes the caller's exit code.
+
+CONFIG_DIR="${XDG_CONFIG_HOME:-$HOME/.config}/linux_post_install"
+
+# Effective platform list (env > notify.env > "telegram").
+notify_platforms() {
+    local p="${NOTIFY_PLATFORM:-}"
+    if [ -z "$p" ] && [ -f "$CONFIG_DIR/notify.env" ]; then
+        p="$(grep -E '^NOTIFY_PLATFORM=' "$CONFIG_DIR/notify.env" | tail -1 | cut -d= -f2-)"
+        p="${p%\"}"; p="${p#\"}"; p="${p%\'}"; p="${p#\'}"
+    fi
+    printf '%s' "${p:-telegram}"
+}
 
 notify_send() {
     local msg="" markdown=0
@@ -26,22 +49,29 @@ notify_send() {
         return 0
     fi
 
-    local tg
-    tg="$(command -v pos-communication-telegram 2>/dev/null)" || \
-        tg="$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")/../bin/pos-communication-telegram"
+    local -a plist
+    IFS=',' read -r -a plist <<< "$(notify_platforms)"
 
-    if [ ! -x "$tg" ]; then
-        warn "notify_send: pos-communication-telegram not found, notification skipped" 2>/dev/null || true
-        return 0
-    fi
+    local p sender
+    for p in "${plist[@]}"; do
+        p="${p// /}"
+        [ -n "$p" ] || continue
+        sender="$(command -v "pos-communication-${p}" 2>/dev/null)" || \
+            sender="$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")/../bin/pos-communication-${p}"
 
-    if [ "$markdown" -eq 1 ]; then
-        "$tg" send "$msg" --parse-mode markdown >/dev/null 2>&1 || {
-            warn "notify_send: telegram send failed, notification skipped" 2>/dev/null || true
-        }
-    else
-        "$tg" send "$msg" >/dev/null 2>&1 || {
-            warn "notify_send: telegram send failed, notification skipped" 2>/dev/null || true
-        }
-    fi
+        if [ ! -x "$sender" ]; then
+            warn "notify_send: pos-communication-${p} not found, notification skipped" 2>/dev/null || true
+            continue
+        fi
+
+        if [ "$markdown" -eq 1 ]; then
+            "$sender" send "$msg" --markdown >/dev/null 2>&1 || {
+                warn "notify_send: ${p} send failed, notification skipped" 2>/dev/null || true
+            }
+        else
+            "$sender" send "$msg" >/dev/null 2>&1 || {
+                warn "notify_send: ${p} send failed, notification skipped" 2>/dev/null || true
+            }
+        fi
+    done
 }