fix(web): statusline tokens, unreachable dot, reachability test spans (#145)

Second review round (spec + standards):
- Polling-off and unreachable render as statusline tokens; the unreachable
  span drops mark-strong so the patina dot never sits next to red text.
- Reachability tests assert on the rendered span, which 'reachable' and
  'unreachable' substrings never could.
- A pass-log query failure now reads as reachable (no evidence rule) instead
  of condemning the browser, and admin.go loses its dead time import.
- startLatestPoller's doc no longer claims the admin page reads the poller;
  AGENTS.md carries the RefuseBackoff rename.

backend/internal/web/admin_lanes.go

@@ -81,6 +81,10 @@ func (h *Handler) lanesView() lanesView {
 	passes, err := h.store.LatestLanePasses()
 	if err != nil {
 		log.Printf("admin lanes: latest passes: %v", err)
+		// No evidence of a lost sidecar reads as reachable, per the same rule
+		// browserReachable applies: a store failure must not condemn the
+		// browser. The empty table already says no Lane has recorded a pass.
+		v.BrowserReachable = true
 		return v
 	}
 	now := time.Now()