engage-desk v3: search every 2 h, three topics a search, Habr without a sign-in, own posts skipped #6201

The owner asked on 2026-10-05 for a more proactive engage desk: it searches too rarely and not on every network (diagnosis: gHashTag/999-multibots-telegraf#3606).

Spec of record: specs/automation/engage-desk.t27, v2 -> v3.

• SEARCH_EVERY_MS 6 h -> 2 h.
• New TOPICS_PER_SEARCH = 3: a search covers three feed topics in rotation, not one.
• PER_SEARCH_PER_NETWORK 1 -> 2, DAILY_CAP_PER_NETWORK 3 -> 5.
• A search is marked when it ends; a start left open (a deploy cut it) is retried after a stale window instead of losing the whole interval.
• The owner's own posts are never commented on.
• Habr is searched without a sign-in (its public search); a Habr proposal says when the pod cannot post there.
• The per-comment yes (ASK_NEEDS_YES) is unchanged.
• Honesty fix: v2's Facebook and Telegram readers were specified but never built in the host (its vendored copy is still v1); READER_NETWORKS says what is built.

Host change follows in gHashTag/999-multibots-telegraf (Refs #3606).