Wire shared MOTD into BBS hub and IRC

Use profullstack.com/motd as the shared Message of the Day across both
the SSH BBS hub and the Ergo IRC server.

- internal/motd: fetch + in-memory cache with background refresh (stdlib
  only); Current() never blocks session start, keeps last value on error.
  Source overridable via AGENTBBS_MOTD_URL.
- hub: append the daily MOTD below the existing welcome/onboarding text.
- IRC: deploy/ergo/refresh-motd.sh pulls /motd into Ergo's ergo.motd and
  rehashes; setup.sh installs it + an ergo-motd.timer (hourly) mirroring
  the ergo-certs timer, with a seeded fallback if the source is offline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/motd/motd.go

@@ -0,0 +1,90 @@
+// Package motd fetches the shared Message of the Day from profullstack.com and
+// caches it in memory, refreshing in the background. The hub MOTD reads the
+// cached value (never blocking session start); a failed fetch keeps the last
+// known value. Set AGENTBBS_MOTD_URL to override the source (empty disables).
+package motd
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+)
+
+// DefaultURL is the canonical shared MOTD endpoint (plain text, CORS-open).
+const DefaultURL = "https://profullstack.com/motd"
+
+var (
+	mu     sync.RWMutex
+	cached string
+)
+
+// Current returns the most recently fetched MOTD, or "" if none has been
+// fetched yet (or fetching is disabled/failing).
+func Current() string {
+	mu.RLock()
+	defer mu.RUnlock()
+	return cached
+}
+
+func set(s string) {
+	mu.Lock()
+	cached = s
+	mu.Unlock()
+}
+
+func fetch(ctx context.Context, url string) (string, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return "", err
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return "", &httpError{resp.StatusCode}
+	}
+	// Cap the read; the MOTD is a short text blob.
+	b, err := io.ReadAll(io.LimitReader(resp.Body, 8<<10))
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(b)), nil
+}
+
+type httpError struct{ code int }
+
+func (e *httpError) Error() string { return "motd: unexpected status " + http.StatusText(e.code) }
+
+// Start fetches the MOTD once (blocking, with a short timeout so a slow source
+// can't stall boot for long) then refreshes every `every` until ctx is done.
+// A url of "" disables fetching entirely.
+func Start(ctx context.Context, url string, every time.Duration) {
+	if url == "" {
+		return
+	}
+	refresh := func() {
+		c, cancel := context.WithTimeout(ctx, 8*time.Second)
+		defer cancel()
+		if s, err := fetch(c, url); err == nil && s != "" {
+			set(s)
+		}
+	}
+	refresh()
+	go func() {
+		t := time.NewTicker(every)
+		defer t.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-t.C:
+				refresh()
+			}
+		}
+	}()
+}