feat(files): provision-user CLI + anonymous public HTTP serving

Lets external services (the TronBrowser extension store) host files on
files.profullstack.com without the interactive `ssh join@` onboarding.

- `agentbbs provision-user --name <h> --pubkey "<ssh key>"`: registers a member
  from an SSH *public* key (account = handle + key fingerprint). Reuses
  SanitizeUsername (same rules as join@) + EnsureUser; Files/SFTP access is free
  for members, so the account can immediately
  `scp … files@host:/public/extensions/<slug>/`. JSON output; refuses on key/
  handle collision. New auth.FingerprintAuthorizedKey() parses an
  authorized_keys line to the same SHA256 fp as a live session key (tested).
- setup.sh: the files.<host> Caddy site now serves the shared /public area as
  unauthenticated, read-only static files (handle_path /public/*), so .crx/.zip
  download links work for anyone — mapping 1:1 to the SFTP path. Non-/public
  paths still hit the auth'd web file manager.
- docs/files.md updated.

Note: not compiled here — repo go.mod requires go 1.26 and this sandbox has
1.22.2; changes pass gofmt parse/format checks. Reuses existing store/auth APIs.

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

docs/files.md

@@ -88,6 +88,45 @@ content-blind.
 | `AGENTBBS_FILES_QUOTA_MB` | `1024` | default per-user workspace quota (MB) |
 | `AGENTBBS_DATA` | `./data` | storage lives under `<data>/files/{users,public}` |
 
+## Provisioning members from a public key (for external services)
+
+Normally members onboard interactively (`ssh join@`). External services that
+want to grant a user file storage without that flow — e.g. the TronBrowser
+extension store letting a publisher upload bundles — can register an account
+directly from an SSH **public** key (an account is just *handle + key
+fingerprint*):
+
+```bash
+agentbbs provision-user --name acme --pubkey "ssh-ed25519 AAAA… acme@dev"
+# or:  --pubkey-file ./id_ed25519.pub
+```
+
+It normalizes the handle with the same rules as `join@` (`SanitizeUsername`),
+fingerprints the key, and `EnsureUser`s the member; Files/SFTP access is then
+available immediately (free for all members). Output is JSON (`{ok, name,
+fingerprint, store_id}`); it refuses if the key already belongs to another
+member or the handle is taken by a different key. The publisher can then:
+
+```bash
+scp dist.crx files@files.profullstack.com:/public/extensions/acme/
+```
+
+## Public files over HTTP (anonymous, read-only)
+
+The web file manager at `files.<host>` requires a login even to download, which
+is wrong for *shared* artifacts (a `.crx` download link must work for anyone).
+So the `files.<host>` Caddy site (generated by `setup.sh`) serves the shared
+`/public` area as **unauthenticated, read-only** static files, mapping 1:1 to
+the SFTP path:
+
+```
+scp x files@files.profullstack.com:/public/extensions/acme/x
+        ->  https://files.profullstack.com/public/extensions/acme/x
+```
+
+Everything outside `/public/*` still falls through to the authenticated web
+manager (private `/me` browsing).
+
 ## Implementation
 
 - `internal/files` — a fully virtual Go SFTP server (`github.com/pkg/sftp` +