fix(packaging): ship the example hub config, and stop leaving /etc/meshbay open

Three defects, found while answering whether installing the .deb would land
where the production server was just moved to by hand.

- **The example config was never packaged.** `build-hub.sh` copied
  `packaging/conf/hub.toml.example` under `if [ -f ]`, and that path does not
  exist in this repo — so every package ever built shipped no example at all
  and said nothing about it. The postinst places no config either, on purpose
  (a shipped hub.toml is overwritten on upgrade; a shipped secret gets run in
  production), which left an installed hub with nothing to copy from. The file
  now exists, documents every key `config.py` reads including the captcha
  `allowed_hosts` the desktop client needs, and the copy is a hard failure
  rather than a silent skip.

- **`/etc/meshbay` was created 0755.** It holds the hub's Ed25519 private key
  and its database password. The file modes protect the contents, but a
  world-listable config directory tells anyone with a shell what a hub keeps
  and where. Now 0750 root:meshbay, in both the deb postinst and the rpm
  scriptlet; the service reads it by group.

- **The rpm would have failed to build on the new file.** `%files` claimed
  nothing under /etc, and rpmbuild refuses an installed file no line claims.
  It now declares the directory and the example, with explicit `%attr` and
  `%config` so an operator's edits become .rpmsave rather than vanishing.

Package modes no longer follow the builder's umask either — the same source
tree produced 775/664 on a machine with umask 002 and 755/644 with 022.
`install -m` sets them.

Verified by building: the deb now carries ./etc/meshbay/ at drwxr-x--- with
hub.toml.example at 0644, and the embedded postinst tightens the directory as
belt and braces rather than as the only thing making it right. The rpm path is
unverified — no rpmbuild on this machine.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014UtzVrzM7e2tG9fSpkR9ML

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