Replace the protobuf hex package (agent.pb.ex) and hand-written validator
(validator.ex) with a pure Gleam implementation: wire format primitives,
all 23 message types, encode/decode functions, and validation merged into
decode. Thin Elixir wrappers in agent.ex preserve the existing struct API.
- Wire format: varint, length-delimited, double (IEEE 754 via FFI), tag encode/decode
- Types: all proto3 message types as Gleam custom types with enums and oneof
- Decode: field-level parsing with integrated validation (UUID, IP, hostname, limits)
- Encode: proto3 default omission, bytes_tree concatenation, map/repeated fields
- Wrappers: Elixir structs with encode/decode delegating to Gleam, enum atom conversion
- Remove {:protobuf, "~> 0.12"} dependency
Reviewed-on: graham/towerops-web#104
Protobuf oneof fields use a single :config key with tagged tuples
like {:dns, %DnsCheckConfig{}} rather than individual keys. The
check_type_config functions were setting e.g. dns: %DnsCheckConfig{}
directly, causing KeyError crashes in Check.__struct__/1.
Reviewed-on: graham/towerops-web#101
