(fix)build: disable proto rule generation, rely on checked-in .pb.go for go-code compatibility

[roychying]

Why?

We want the smoothest DX for consuming the OSS submitqueue repo in go-code. Today go-code's go_repository uses build_file_generation = "on", so its Gazelle regenerates BUILD files for the vendored repo. This causes conflicts and mismatches, and every version bump risks new patching.

The root issue is that OSS carries proto types twice: as checked-in .pb.go in protopb/, and as Bazel-generated code via go_proto_library in proto/. Removing the Bazel proto rules in favor of the checked-in .pb.go fixes several things at once:

• Eliminates duplicate proto definitions. The two generation paths share the same importpath, which caused the duplicate-importpath ambiguity and forced the gazelle:resolve workarounds. Collapsing to a single source removes that whole class of conflict.
• Serves non-Bazel consumers. The Bazel-generated code only exists inside a Bazel build graph. Anyone consuming submitqueue via plain go build/go get (including OSS users) can only use the checked-in .pb.go, so that path must work regardless. The Bazel proto rules are a redundant second path that serves no one the checked-in files don't already serve.

What?

Disable Gazelle proto rule generation and remove all proto compilation rules, relying solely on checked-in .pb.go files.

• Add # gazelle:proto disable_global to prevent Gazelle from regenerating proto rules on future make gazelle runs
• Remove the gazelle:resolve directives that are no longer needed
• Remove proto_library, go_proto_library, and alias go_library rules
• Keep exports_files for make proto codegen (tool/proto/BUILD.bazel)

Test Plan

• make build - pass
• make test - 65/65 unit tests pass (integration/e2e tests fail due to Docker dependency, pre-existing)
• bazel build //tool/proto:generated - proto codegen still works
• make gazelle - zero warnings, no duplicate-importpath ambiguity
• Validated in go-code with --override_repository pointing at this branch: all 71 targets build, all 20 tests pass across stovepipe, submitqueue/gateway, and submitqueue/orchestrator

Issue

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