Extract hydra-proto crate for shared gRPC code generation

[Ericson2314]

Both hydra-builder and hydra-queue-runner had near-identical
build.rs files that compiled the same .proto file and generated
the same version constant. This consolidates that into a single
hydra-proto crate with feature flags:

• client — generates tonic client stubs (used by hydra-builder)

• server — generates tonic server traits (used by hydra-queue-runner)

• db — From impls bridging proto enums to db::models types

Proto ↔ native type conversions are also consolidated here:

• ProtoStorePath moved from shared::proto, now depends on
  harmonia-store-core directly rather than nix-utils

• Pressure/PressureState — both components now use the generated
  proto types directly, eliminating duplicate structs and field-by-field
  conversion boilerplate

• PresignedUploadOpts, ConfigUpdate — queue-runner now uses the
  proto types directly instead of maintaining trivial local wrappers

• BuildResultState — restructured as a nested enum wrapping
  hydra_proto::BuildResultState with extra Aborted/Cancelled
  variants for queue-runner-internal states

• RelativeStorePath — new proto message type making the store path
  vs sub-path split explicit on the wire; From/TryFrom conversions
  in hydra-proto bridge to store_path_utils::RelativeStorePath

• BuildProduct.path changed from opaque string to structured
  RelativeStorePath message

Also:

• Groups all local workspace crates under [workspace.dependencies]
  with workspace = true references instead of relative paths

• Updates the architecture doc with descriptions of each shared crate