docs: add agent instructions with public repository confidentiality rules

AGENTS.md

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+# mParticle Flutter SDK - Agent Instructions
+
+A Flutter plugin that exposes the mParticle API on iOS, Android and web. `README.md` covers integration.
+
+## This is a PUBLIC repository
+
+Everything pushed here is world-readable and permanent: code, comments, commit messages, branch
+names, PR titles and descriptions, review comments, test names, fixtures and assets. Closing a
+PR, renaming a branch or force-pushing does not take any of it back, because GitHub keeps a
+closed PR's original branch name and commits. The first push is the one that counts.
+
+Never include:
+
+- Partner, client or advertiser names, or any detail that could identify one (account, tenant
+  or campaign IDs, deal terms, integration specifics). Say "a partner" or use a synthetic
+  example, in branch names too.
+- Partner brand assets: logos, screenshots or copy. Demo and sample content uses fictional
+  brands.
+- Internal service, system or class names, internal hostnames, dashboards, or links to private
+  repos and tickets.
+- Backend detail, especially how a payload is validated server-side. Describe what the SDK
+  sends and receives, in partner-facing terms, and refer to a server change generically ("to
+  match the server contract").
+
+Before each push, PR, comment or reply, read the exact outgoing text, branch name included. If
+you are unsure whether a detail is safe, leave it out and ask privately. Do not publish first
+and edit it out later.

CLAUDE.md

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+# mParticle Flutter SDK - Agent Instructions
+
+<!-- Claude Code reads CLAUDE.md, not AGENTS.md. Keep all guidance in AGENTS.md. -->
+
+@AGENTS.md