feat(files): SFTP member storage — private workspaces + shared public area + mgmt TUI

Implements M4 (Files). A fully virtual Go SFTP server (pkg/sftp + crypto/ssh,
no OS users) wired as an "sftp" subsystem on the existing :22 wish listener, so
members reach their files with their login key:

    sftp files@bbs.profullstack.com     # scp/rsync ride the same endpoint

Identity is the SSH key (the username is conventional/ignored). Two areas per
session: a private, quota-limited /me workspace and a single shared public file
area /public (old-school BBS file area; world-read, members-only write by
default, operator-moderated). This reverses the old NG1 "no sharing" boundary in
favour of one sanctioned, inspectable sharing surface (PRD §9.3 amended).

internal/files:
- backend.go  service, layout, quota/usage, live-session registry, operator API
- fs.go       per-session virtual FS; resolve() is the single security
              chokepoint (area confinement + symlink-escape guard) + pkg/sftp
              request handlers
- server.go   subsystem handler: key auth -> member session -> request server,
              with byte metering and force-disconnect
- tui.go      in-BBS member browser (hub plugin "Files")
- admin.go    operator management TUI: sessions, workspaces/quotas, public area

Operator console: ssh sftp@<host> (allowlist-gated; sftpadmin@/filesadmin@
aliases) — list/disconnect sessions, set per-user quotas, revoke SFTP access,
toggle public write, moderate the public area.

store: files_access (per-user quota override + revoked) and files_settings
(public-write mode) tables + methods. main.go wiring guarded by AGENTBBS_FILES
(+ AGENTBBS_FILES_QUOTA_MB, default 1 GiB). Route names reserved.

Tests (incl -race): path traversal/confinement, symlink-escape rejection,
public-write ACL, quota enforcement, usage accounting, and an end-to-end run
against a real SFTP client. Docs: docs/files.md; PRD §5.3/§5.3.1/§9.3 + README
updated.

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

internal/auth/auth.go

@@ -69,6 +69,11 @@ var NewsNames = map[string]bool{"news": true}
 // an interactive TUI with a PTY, or a JSON bot mode with a command/no PTY.
 var MailNames = map[string]bool{"mail": true}
 
+// FilesAdminNames route an operator into the SFTP server management TUI. Members
+// transfer files via the "sftp" subsystem (sftp files@<host>); this interactive
+// route is the operator console and is gated by the admin allowlist.
+var FilesAdminNames = map[string]bool{"sftp": true, "sftpadmin": true, "filesadmin": true}
+
 // GameNames are usernames that route to AgentGames: the line-delimited-JSON
 // agent-vs-agent match protocol (PRD §5.2). `play@` stays a guest hub alias.
 var GameNames = map[string]bool{"game": true, "games": true}
@@ -103,6 +108,10 @@ func IsNewsName(u string) bool { return NewsNames[strings.ToLower(u)] }
 // IsMailName reports whether the SSH username requests the AgentMail client.
 func IsMailName(u string) bool { return MailNames[strings.ToLower(u)] }
 
+// IsFilesAdminName reports whether the SSH username requests the SFTP server
+// management TUI (operator-gated).
+func IsFilesAdminName(u string) bool { return FilesAdminNames[strings.ToLower(u)] }
+
 // systemReserved are names that don't drive an SSH route but would still
 // collide with a per-user subdomain (<name>.<host>), the agent route, or common
 // infra hostnames — so members may not claim them as account names.
@@ -119,7 +128,8 @@ var systemReserved = map[string]bool{
 func IsReservedName(name string) bool {
 	n := strings.ToLower(name)
 	if GuestNames[n] || PodNames[n] || JoinNames[n] || DomainNames[n] || AdminNames[n] ||
-		TorURLNames[n] || TorIRCNames[n] || TorNames[n] || IRCNames[n] || NewsNames[n] || systemReserved[n] {
+		TorURLNames[n] || TorIRCNames[n] || TorNames[n] || IRCNames[n] || NewsNames[n] ||
+		MailNames[n] || FilesAdminNames[n] || systemReserved[n] {
 		return true
 	}
 	return strings.HasPrefix(n, "video-") // video-<code> call routes