feat(members): member directory + store-and-forward messaging

A members-only hub plugin (the BBS "who") plus user-to-user messaging:

- internal/store: messages table + SendMessage/Inbox/UnreadCount/MarkRead, and
  OnlineUsers (open sessions) for presence. MarkRead is recipient-scoped so a
  member can only clear their own mail.
- plugins/members: directory with online dots + last-seen, a finger-style
  profile view, a minimal compose box, and an inbox that marks read on open.
- ssh msg@host <user> [text]: scriptable CLI to leave a note (body from args or
  stdin), mirroring the existing finger route; "msg"/"message" are reserved.
- hub: "N unread" badge on login (hubMOTD). plugin.Context gains Host for member
  homepage URLs.

Extends the existing finger@ behavior (ssh <name>@host) rather than replacing it.

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

internal/plugin/plugin.go

@@ -22,6 +22,9 @@ type Context struct {
 	DataDir string
 	// AssetsDir is the read-only platform assets tree (wads, binaries).
 	AssetsDir string
+	// Host is the BBS hostname (e.g. bbs.profullstack.com), for building
+	// member homepage URLs (https://Host/~name) and similar links.
+	Host string
 }
 
 // Plugin is the only integration point between a feature and the hub.