Read RSS and Atom feeds, and speak about them only when asked (#258)

internal/rss parses RSS 2.0 and Atom, and polls each configured feed on its own
interval; internal/webfetch is the one door either of them uses to touch the
network. The poller writes items as notes with source "rss:<feed>" and nothing
else: the answer path reads them back when he asks "что нового в лентах?", and
nothing is announced on arrival. A feed that dispatched would be a nag, which is
why the plan's breaking-news rule was left out rather than built.

webfetch is where the limits live, as code rather than a paragraph: http(s)
only, an allowlist (the configured feeds' hosts) and a denylist, a 2 MiB body
cap, a 3-redirect cap, one request per host per second, and a refusal to connect
to any private address — checked in the dialer's Control hook so it holds for
every resolved address and every redirect hop, not just for a literal IP.

Off unless configured: no "feeds" block, no poller, no outbound request. How far
a feed was read is a config fact (rss:latest:<name>), so a restart does not
re-note yesterday's headlines.

cmd/mavend/voice.go

@@ -82,6 +82,11 @@ type reactiveHandler struct {
 	replier  voice.Replier
 	now      func() time.Time
 
+	// feedsOn — whether any RSS feed is configured (config.Feeds). It changes
+	// only what she SAYS when asked and nothing is there: "ленты не настроены"
+	// instead of "ничего нового", which are different truths.
+	feedsOn bool
+
 	weatherProvider weather.Provider
 	weatherLocation string // default location for weather queries