feat: GoReleaser/Homebrew distribution; consolidate proto package to mcp

Release & distribution:
- Add .goreleaser.yaml (6-platform binaries, archives, checksums, changelog)
  and a Homebrew cask published to the-protobuf-project/homebrew-tap
- Rewire the release workflow to run GoReleaser, replacing the hand-rolled
  build/archive/changelog/release steps (BSR push retained)

Stop publishing language packages:
- Remove the PyPI (grpc-mcp-gateway-protos) and crates.io (mcp-protobuf) publish
  jobs, the mcp/protobuf/python and mcp/protobuf/rust packages, and their
  proto-python/proto-rust CI checks. Non-Go clients generate these types from
  the published Buf module (buf.build/the-protobuf-project/mcp).
- Vendor the mcp.protobuf Python types into examples/proto/generated/python so
  the Python examples keep working without the published package.

Proto package rename:
- Rename the annotation package mcp.protobuf -> mcp to match the published Buf
  module; regenerate Go/Python/Rust/C++ and update docs.

Fixes:
- Pin the Buf python plugin to v34.1 so gencode (7.34.1) matches the available
  protobuf runtime, fixing the cross-version runtime error.
- Detect the MCPProgress oneof under either `mcp` or legacy `mcp.protobuf`,
  restoring the counter example's streaming tool.

Author: oh-tarnished

examples/proto/generated/python/google/__init__.py

@@ -0,0 +1 @@
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)

examples/proto/generated/python/google/api/__init__.py

@@ -0,0 +1 @@
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)

examples/proto/generated/python/google/api/resource_pb2.py

@@ -0,0 +1 @@
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)