Add default contribution guidance and a pull request template

[Scriptception]

Problem

Public repositories without local guidance do not inherit a useful organization-wide contribution path, so contributors cannot predict setup, scope, testing, review, or change-log expectations.

User outcome

A contributor can prepare a focused, testable change and understand when repository-specific instructions override the default.

Scope

• Add a default CONTRIBUTING guide with discovery, issue, branch, testing, documentation, and review expectations.
• Add a concise pull-request template focused on outcome, evidence, risks, and screenshots where applicable.
• Explain precedence of repository-specific contributor files.
• Link the guidance from the organization profile.

Non-goals

• Imposing one build command on every repository.
• Replacing product-specific development documentation.

Acceptance criteria

• Repositories without local files inherit actionable defaults.
• The pull-request template requests user outcome, test evidence, risk, and relevant visual proof.
• Guidance distinguishes required policy from examples.
• Override and escalation behavior is documented.

Validation

• Preview a pull request in a repository with and without local overrides.
• Ask a new contributor to follow the default against a representative documentation change.

Relationships

• Complements default issue forms and governance guidance.
• Product repositories remain authoritative for technical commands.