The previous translation of the dovecot 2.3 plugin/namespace blocks didn't
survive doveconf parsing or the runtime check for virtual namespaces:

- `acl = vfile` is a named-list-filter in 2.4. The driver moved to
top-level `acl_driver`, and per-identifier defaults live in
`acl <identifier> { acl_rights = ...; }` filters keyed on the ACL id
(owner/anyone/user=...). The filter name itself is parsed by
acl_identifier_parse, which is why earlier attempts with `acl vfile`
or `acl primary` failed with "Invalid identifier".
- `acl_shared_dict` was removed; the bot subscribes to shared paths
explicitly so we don't need it.
- Per-namespace `location = "driver:path:opts"` was split into separate
mail_driver / mail_path / mail_index_path settings.
- The virtual namespace also needs `mailbox_list_layout = fs` to override
the global Maildir++ - otherwise dovecot 2.4 refuses to initialise the
namespace, which broke both LMTP delivery and IMAP login.

Verified on neoprism with `doveconf -n` and a live LMTP delivery.

Dovecot 2.4 (now in nixpkgs) removed services.dovecot2.extraConfig and
mailPlugins. Translate the plugin/namespace blocks into the structured
settings tree, hoist ACL plugin keys to top-level (no more `plugin {}`
section), and update the variable-substitution syntax (%n -> %{user |
username}, %u -> %{user}).

Also drop the explicit master ref on nixos-mailserver since the project
no longer uses that branch name as default tracking.

claude is already provided via tools.s (and the default tools include
s now), so drop the explicit claude entry and add s for direct access.

Use the new wrapper plugins option to install the GouvernAI plugin from
GitHub, providing the /guardrails command and gouvernai flow-state skill.

Allow declaratively adding extra Claude Code plugin directories alongside
the bundled one. Each entry becomes a repeatable --plugin-dir flag.

Add MegolmEvent callback that warns the user when a message can't be
decrypted and requests the missing room key. Also process invited_rooms
from the initial sync so invites that arrived while the bot was offline
are accepted instead of being silently ignored.