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>

setup.sh

@@ -306,6 +306,13 @@ AGENTBBS_NEWS_TLS_KEY=${DATA_DIR}/news-tls/privkey.pem
 # AGENTBBS_NEWS_ADDR=127.0.0.1:1119
 # AGENTBBS_NEWS_TLS_ADDR=:563
 # AGENTBBS_NEWS_GROUPS=pfs.announce:Announcements,pfs.general:General,pfs.agents:Agents
+
+# Files (SFTP): member storage on the same :22 listener (sftp files@${DOMAIN}).
+# Private per-user workspace (/me, quota-limited) + a shared public area
+# (/public). Storage lives under \${AGENTBBS_DATA}/files. Operators manage it via
+# \`ssh sftp@${DOMAIN}\`. Set AGENTBBS_FILES=0 to disable. See docs/files.md.
+# AGENTBBS_FILES=1
+# AGENTBBS_FILES_QUOTA_MB=1024
 ENV
   chmod 0640 "$ENV_DIR/agentbbs.env"
 fi