docs: add AGENTS.md

AGENTS.md

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+This codebase is a tiered, caching, protocol-aware, pass-through HTTP proxy. The cache is designed for software
+engineering machines, in CI, interactive in the cloud, or locally.
+
+The cache server is tiered, in that it runs locally on workstations, but backs onto remote instances of the cache
+server. If the local instance doesn't have an object, it will automatically fall back to the remote instance.
+
+Logically, it's broken up into two main components - cache backends, and caching strategies. Cache backends are object
+stores with some extra metadata and per-object TTLs. Strategies are protocol-aware caching mechanisms. For example, the
+github-releases strategy knows how to retrieve and cache private and public GitHub releases artefacts. Another strategy
+example is for Git, where the proxy is aware of the Git protocol and uses it to reduce the impact on upstream Git
+servers.
+
+The way the two components interact is that the strategies have access to a Cache implementation, which they use to
+cache and restore data. Strategies are not limited to using the Cache though; for example the Git strategy could 
+keep a local bare checkout of upstream Git repositories and serve packs from the repo directly.
+
+The codebase uses Hermit to manage toolchains. It is written in Go, and uses Just for running common tasks.