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

lib/flags.sh

@@ -0,0 +1,60 @@
+# ── Feature flag store ─────────────────────────────────────────
+# System-wide flags: one file per flag in $FLAGS_DIR.
+#   presence = flag set, file content = optional value.
+# Sourced by scripts; reads are plain file ops, writes use sudo.
+# Override FLAGS_DIR via environment for testing.
+FLAGS_DIR="${FLAGS_DIR:-/usr/local/share/linux_post_install/flags}"
+
+# flag_set <name> [value] — mark a flag as set (green), optionally with a value
+flag_set() {
+    local name="$1" value="${2:-}" tmp
+    run sudo install -d -m 755 "$FLAGS_DIR"
+    if [ -n "$value" ]; then
+        tmp=$(mktemp)
+        printf '%s' "$value" > "$tmp"
+        run sudo install -m 644 "$tmp" "$FLAGS_DIR/$name"
+        rm -f "$tmp"
+    else
+        run sudo touch "$FLAGS_DIR/$name"
+    fi
+}
+
+# flag_clear <name> — remove a flag
+flag_clear() {
+    run sudo rm -f "$FLAGS_DIR/$1"
+}
+
+# flag_is_set <name> — 0 if flag is set, 1 otherwise
+flag_is_set() {
+    [ -f "$FLAGS_DIR/$1" ]
+}
+
+# flag_value <name> — print the stored value (empty when unset or valueless)
+flag_value() {
+    local f="$FLAGS_DIR/$1"
+    [ -f "$f" ] && cat "$f"
+}
+
+# flag_list — print the names of all set flags, one per line
+flag_list() {
+    [ -d "$FLAGS_DIR" ] || return 0
+    for f in "$FLAGS_DIR"/*; do
+        [ -f "$f" ] || continue
+        printf '%s\n' "$(basename "$f")"
+    done
+}
+
+# flag_status <name> — print a human-readable status line
+flag_status() {
+    local name="$1" v
+    if flag_is_set "$name"; then
+        v=$(flag_value "$name")
+        if [ -n "$v" ]; then
+            echo "set: $name=$v"
+        else
+            echo "set: $name"
+        fi
+    else
+        echo "unset: $name"
+    fi
+}