P4A Build Policy

Use when assembling a GitHub-hosted policy repository so the P4A (Policies 4 Agents) platform can accept it as a submission. Covers the two accepted layouts — the unified model (one root with Makefile, Cargo.toml, src/lib.rs) and the split model (a definition root with gcl.yaml + exchange.json + Makefile and a separate implementation root with Cargo.toml + Makefile + src/lib.rs) — the pdk dependency floor (version >= 1.8, including Cargo workspace inheritance via pdk = { workspace = true }), the public-repo requirement, subPath addressing for policies nested in a monorepo, optional files (definition/gcl.yaml, icon.png/icon.svg), the catalog metadata (name, description, ca…