Add AI policy, AGENTS.md and SpecKit configuration

[freakboy3742]

An initial set of Generative AI configurations for Briefcase. This is being added following the discussion on beeware/beeware#621, following the adoption of beeware/.github#328

Adds:

• An AGENTS.md to guide generative AI interactions
• A Spec Kit project constitution
• An update CONTRIBUTING.md that references the new policies
• A copy of the AI policy in the contribution guide.

AGENTS.md should be picked up by any code agent.

The Spec Kit constitution will be used if you follow the Spec Kit installation instructions (e.g., specify init . --force --ai opencode).

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Opus 4.7