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.

deploy/README.md

@@ -36,6 +36,37 @@ present (see `.dockerignore`).
 | `/var/lib/maven` (volume)  | encrypted db at rest                    |
 | `/dev/shm` (tmpfs)         | decrypted db working copy (RAM only)    |
 
+## Reading the outside world (off by default)
+
+`mavend.json` ships without a `feeds` block, which means no RSS/Atom feed is
+fetched and no outbound request is made. Switching it on is adding the block:
+
+```json
+"feeds": {
+  "poll_interval": "30m",
+  "max_items": 5,
+  "max_age": "24h",
+  "sources": [
+    { "name": "habr", "url": "https://habr.com/ru/rss/best/daily/",
+      "category": "технологии", "exclude": ["реклама"] }
+  ]
+}
+```
+
+What it does and does not do:
+
+- items are written as notes with source `rss:<name>`, visible on `/dash`;
+- **nothing is announced.** She reads them back when asked — "что нового в
+  лентах?", "что нового по технологиям?" — and never on arrival. There is no
+  severity or channel knob here on purpose;
+- the fetcher is allowlisted to the hosts of the configured feeds, plus any
+  `allow_hosts`. It refuses non-http(s) schemes and every private address
+  (loopback, the LAN, the `10.42.0.0/24` wg range, cloud metadata). It caps the
+  response at 2 MiB and redirects at 3, and makes at most one request per host
+  per second. See `internal/webfetch`;
+- how far each feed was read is stored as a config fact `rss:latest:<name>`, so
+  a restart does not re-note yesterday's headlines.
+
 ## Not yet verified / host-dependent
 
 This stack is correct-by-construction but has **not been build-tested here**