files: per-user public at ~name/public; home = member directory

Re-model the web file host as a file server (not a website host):

- Drop the misnamed /site area. A member's public files are now their
  /me/public subfolder (unix ~/public), served anonymously at
  ~<name>/public. The rest of /me stays private; only ~name/public is
  ever exposed. Bare /~name redirects to /~name/public/.
- The root / is now a directory of ALL members, each linked to their BBS
  site (https://<bbs-host>/~name via WebConfig.SiteBase) AND their public
  files here (~name/public). No longer hides empty members.
- Sites/homepages stay on the BBS — files.<host> only links to them.
- Usage gauge is just /me again (which includes /me/public).

setup.sh + docs updated; tests cover ~name/public browse/download, the
bare-~name redirect, empty-member empty-listing, /public-only exposure,
and traversal confinement.

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

cmd/agentbbs/fileweb.go

@@ -23,7 +23,10 @@ func (a *app) startFilesWeb() {
 	}
 	addr := env("AGENTBBS_FILES_WEB_ADDR", "127.0.0.1:8092")
 	title := env("AGENTBBS_FILES_WEB_TITLE", "files."+strings.TrimPrefix(a.host, "bbs."))
-	h := a.files.WebHandler(files.WebConfig{Authenticate: a.filesWebAuth, Title: title})
+	// Member homepages ("sites") live on the BBS host; the ~user directory links
+	// each member to https://<bbs-host>/~name alongside their public files here.
+	siteBase := env("AGENTBBS_FILES_WEB_SITE_BASE", "https://"+a.host)
+	h := a.files.WebHandler(files.WebConfig{Authenticate: a.filesWebAuth, Title: title, SiteBase: siteBase})
 	srv := &http.Server{Addr: addr, Handler: h, ReadHeaderTimeout: 10 * time.Second}
 	go func() {
 		log.Info("files web listening", "addr", addr)