Prepare golangci-lint ecosystem proposal

[denisvmedia]

Problem

teststyle is now usable as a standalone analyzer and as a golangci-lint
module plugin, but it has not yet gone through a dedicated ecosystem-readiness
pass.

Goal

Prepare teststyle for a public golangci-lint ecosystem proposal once the
repository has had enough usage feedback.

Scope

• Review diagnostic names, messages, categories, and default severity for long-term stability.
• Validate the module plugin against current golangci-lint custom build expectations.
• Add any missing compatibility notes for projects using .custom-gcl.yml and .golangci.yml.
• Decide whether the project should propose inclusion in golangci-lint itself, stay as a documented module plugin, or both.
• Prepare the issue/PR/proposal text for the chosen upstream or ecosystem target.

Acceptance Criteria

• The recommended ecosystem path is documented.
• The proposal text is drafted with motivation, rule set, configuration, and examples.
• The proposal references a stable teststyle release and passing CI.