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>

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/livekit/protocol v1.46.0
 	github.com/livekit/server-sdk-go/v2 v2.16.6
 	github.com/pion/webrtc/v4 v4.2.15
+	github.com/pkg/sftp v1.13.10
 	golang.org/x/crypto v0.50.0
 	golang.org/x/term v0.42.0
 	modernc.org/sqlite v1.52.0
@@ -62,6 +63,7 @@ require (
 	github.com/jxskiss/base62 v1.1.0 // indirect
 	github.com/klauspost/compress v1.18.4 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
+	github.com/kr/fs v0.1.0 // indirect
 	github.com/lithammer/shortuuid/v4 v4.2.0 // indirect
 	github.com/livekit/mageutil v0.0.0-20250511045019-0f1ff63f7731 // indirect
 	github.com/livekit/mediatransportutil v0.0.0-20260521165806-8004f10ad0c5 // indirect