chore: add AI contribution policy

[dustinbyrne]

Problem

PostHog's main repo has an AI contributions policy and PR-template agent context section. The SDK and agent-extension repos should set the same expectations so AI-assisted contributions are easier to review consistently.

Changes

• Add AI_POLICY.md at the repository root.
• Add an Agent context section to the PR template.
• Adapt the proof-of-work guidance for SDK and agent-extension projects by using repo-neutral testing guidance and calling out example-project smoke tests where applicable.

How did you test this code?

• Verified the markdown changes locally.
• Ran git diff --check.
• Verified the PR template contains exactly one ## 🤖 Agent context section.

Checklist

• Accounted for backwards compatibility of any changes (no breaking changes!)
• Documentation-only change

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Pi was used to apply the same policy/template update across the SDK and agent-extension repositories. The change was prepared in isolated git worktrees to avoid disturbing existing local branches, then reviewed for duplicate Agent context sections and markdown whitespace issues.