Add a `dotnet-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 .NET 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/dotnet-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: dotnet-service
# @policy title: .NET service (src/tests layout, dotnet test)
# @policy summary: A .NET service. The agent may change project sources and tests, and must keep
#   `dotnet build` and `dotnet test` green. Tree-wide build props, the NuGet source
#   configuration, production appsettings and publish profiles stay off-limits.
# @policy stack: dotnet, csharp, nuget, xunit
# @policy author: your-github-handle
# @policy blocks: Directory.Build.props, nuget.config, src/Api/appsettings.Production.json, src/Api/Properties/PublishProfiles/prod.pubxml, .github/workflows/release.yml
# @policy allows: src/Api/Controllers/CheckoutController.cs, src/Domain/Subscription.cs, tests/Api.Tests/CheckoutTests.cs, src/Api/Api.csproj, README.md
version: 2
task_type: feature-work

allowed_paths:
  - "src/**"
  - "tests/**"
  - "*.sln"
  - "README.md"
  - "CHANGELOG.md"

forbidden_paths:
  # Directory.Build.props / Directory.Packages.props are imported by EVERY project
  # below them, including projects with no file in this diff. One line here changes
  # the compilation of code the reviewer never opened — invisible blast radius.
  - "Directory.Build.props"
  - "Directory.Build.targets"
  - "Directory.Packages.props"
  - "**/Directory.Build.*"
  # nuget.config decides where packages come from. Redirecting a source is a supply
  # chain change that looks like configuration.
  - "nuget.config"
  - "NuGet.config"
  - "**/nuget.config"
  # Inside src/** which is otherwise allowed: production configuration and publish
  # profiles configure the deployed system and are never loaded by `dotnet test`.
  - "**/appsettings.Production.json"
  - "**/appsettings.Prod*.json"
  - "**/PublishProfiles/**"
  - ".github/**"
  - "Dockerfile*"
  - "**/.env*"
  - "**/*secret*"

max_files_changed: 15
required_checks:
  - "dotnet build --warnaserror"
  - "dotnet test --no-build"

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 .NET service that exception is:

**/appsettings.Production.json and **/PublishProfiles/** — both sit under
src/**, which you want allowed. Neither is loaded by dotnet test, so the required check
that gates the change cannot see a mistake in either.

The .NET trap worth a comment in the file

Directory.Build.props is the one to get right — it applies tree-wide by
directory position, not by reference, so nothing in a .csproj diff hints that it applies.
Same trap in reverse for central package management: with Directory.Packages.props in
play, a version bump moves every project at once.

Acceptance criteria

• signetry_core/policies/dotnet-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.