Add a `go-service` policy to the policy registry

[bkd-dotcom]

What this is

signetry-core ships a policy registry: a directory of YAML files, one per repository
shape, that says which paths an AI agent may touch for a given task and which it may not.
Six ship today (signetry_core/policies/):
python-library, node-service, monorepo-service, docs-only, dependency-bump,
ci-workflow-fix. A Go service is missing.

This is the highest-leverage thing you can contribute without touching the kernel: one new
file, no Python, and no test to write — tests/test_policy_registry.py is parametrized
over every file in that directory, so your policy is validated the moment it exists.

The deliverable

One file: signetry_core/policies/go-service.yaml. Copy
python-library.yaml
as your starting shape — the # @policy header comments are load-bearing metadata, not
decoration.

A starting point, not a spec to type in verbatim — argue with it, the reasoning is the
contribution:

# @policy id: go-service
# @policy title: Go service (cmd/internal layout, go test)
# @policy summary: A Go service laid out as cmd/ + internal/. The agent may change handlers, business
#   logic and tests, and must keep `go test ./...` and `go vet` green. Generated code,
#   migrations, the container image and CI stay off-limits.
# @policy stack: go, go-modules, gotest
# @policy author: your-github-handle
# @policy blocks: internal/db/migrations/0007_add_index.sql, internal/api/api.gen.go, Dockerfile, .github/workflows/release.yml, deploy/k8s/prod.yaml
# @policy allows: cmd/server/main.go, internal/handler/checkout.go, internal/handler/checkout_test.go, go.mod, README.md
version: 2
task_type: feature-work

allowed_paths:
  - "cmd/**"
  - "internal/**"
  - "pkg/**"
  - "go.mod"
  - "go.sum"
  - "README.md"
  - "CHANGELOG.md"

forbidden_paths:
  # A file ending .gen.go / .pb.go is the OUTPUT of a generator. Hand-editing it
  # makes the checked-in code disagree with the source it was generated from, and
  # the next `go generate` silently reverts the fix.
  - "**/*.gen.go"
  - "**/*.pb.go"
  - "**/zz_generated*.go"
  # Inside internal/** which is otherwise allowed: a migration is applied once and
  # unapplied by hand at 3am. Schema change is a human decision, not agent scope.
  - "internal/db/migrations/**"
  - "**/migrations/**"
  - ".github/**"
  - "Dockerfile*"
  - "deploy/**"
  - "**/.env*"
  - "**/*secret*"

max_files_changed: 15
required_checks:
  - "go build ./..."
  - "go vet ./..."
  - "go test ./..."

policy_owner: your-team
policy_version: "1.0"

The part people get wrong

Your blocks list must include at least one path that sits inside your own
allowed_paths. Carving an exception out of a directory you otherwise own is the whole
skill. For a Go service that exception is:

internal/db/migrations/** — internal/** is allowed wholesale because that's
where the service actually lives, but a migration is applied once and rolled back by hand
under pressure. Schema change is a human decision.

The Go trap worth a comment in the file

go.sum changing while go.mod doesn't is a dependency swap wearing a checksum
update's clothing, and //go:embed pulls a file into the binary without that file
looking dangerous — an embedded template or cert bundle ships in the artifact. Decide
deliberately whether go.sum belongs in your allowlist and say why in a comment.

Acceptance criteria

• signetry_core/policies/go-service.yaml exists; filename matches @policy id.
• blocks and allows each list 3–4 realistic paths for this stack, and don't overlap.
• At least one blocks entry is inside allowed_paths.
• Every forbidden_paths entry that isn't self-evident carries a comment saying why,
  not just that it is. python-library forbids conftest.py at any depth and the comment
  explains it executes at collection time on every developer machine — that is the bar.
• pytest tests/test_policy_registry.py is green. The suite proves each claimed block is
  actually refused by the real evaluate_contract, so a policy that misleads adopters
  fails CI rather than shipping.
• If the policy permits something risky, it says so in a caution: — signetry init
  prints it at adoption time. A risky policy with no caution gets sent back; a risky
  policy that's honest is fine.

Getting started

The full numbered walkthrough, including what makes a policy worth merging, is in
docs/site/policy-registry.md → "Contributing a policy".

git clone https://github.com/Signetry/core && cd core
uv sync
uv run pytest tests/test_policy_registry.py -q
uv run signetry policies          # your entry should appear here once the file exists

Comment to claim it — one policy per contributor so nobody's work gets duplicated. Happy to
review a half-finished scope list; the reasoning matters more than the YAML.