nix/vm: persist AT Protocol state via 9p and create default label definitions #8

Add 9p shared directory for AT Protocol state (PLC/PDS/Jetstream data and
owner DID) so that identities survive VM reboots even when the qcow2 disk
is deleted.

Key changes:
- Bootstrap script handles existing accounts (login fallback when
createAccount fails with persisted PDS data)
- Creates default label definitions (wontfix, good-first-issue, duplicate,
documentation, assignee) via com.atproto.repo.putRecord
- PostgreSQL (PLC) backed up via pg_dump on shutdown, restored via psql
on boot only when the database is empty (qcow2 was deleted)
- PDS_HOSTNAME set to "localhost" (not "localhost:port") to avoid
https:// URL generation that breaks OAuth loopback validation
- PDS/Jetstream data synced via mkDataSyncScripts
- Owner DID written to both VM disk and 9p mount for host-side access
- Use dev@example.com and tangled-dev.test handle for bootstrap account

Signed-off-by: Alessio Caiazza <code.git@caiazza.info>