feat: add feature-flag subsystem with on-demand features install

- Move autostart.sh from bin/ to features/ so re-installs never reset it
- Add lib/flags.sh (flag_set/clear/is_set/value/list/status) + flag-reader/set/clear CLIs
- install.sh --feature copies features/* to /usr/local/bin with overwrite prompt, sets flags
- postinstall.sh enables autostart.service only when the autostart flag is set
- Document features & flags in README, DEV.md, AGENT_Context_Project.md

bin/flag-set

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+set -euo pipefail
+source "$(dirname "$0")/../lib/common.sh" 2>/dev/null || source "$(dirname "$0")/common.sh"
+source "$(dirname "$0")/../lib/flags.sh" 2>/dev/null || source "$(dirname "$0")/flags.sh"
+
+usage() {
+    cat <<EOF
+Usage: flag-set <name> [value]
+
+Set a feature flag (green), optionally storing a value.
+Writes to $FLAGS_DIR (requires sudo).
+EOF
+    exit 0
+}
+
+case "${1:-}" in
+    -h|--help|"") usage ;;
+esac
+
+flag_set "$1" "${2:-}"
+ok "flag set: $1"