docs(agents.md): adds agents.md to help guide development

AGENTS.md

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+# AGENTS.md
+
+## Critical: Blast Radius
+
+This binary is used as the entrypoint for several Docker images. A bug here
+affects every service that depends on it, and rolling out a fix requires
+rebuilding and redeploying those images. Be extremely cautious with
+changes.
+
+## Env Var Precedence
+
+Vault values overwrite Consul values at the same key. Pre-existing host env
+vars are never overwritten. Vault skips the `global/env_vars` and
+`services/{name}/env_vars` paths in `dev` and `test` environments (the
+`devEnv` guard in `config.go:VaultPaths()`).
+
+## Variable Expansion in Values
+
+Loaded values are passed through `os.Expand` (`env.go:47`). A Consul/Vault
+value containing `$FOO` or `${FOO}` will be expanded against other loaded
+vars and the host environment. This is intentional but easy to miss — values
+with literal `$` must avoid this syntax or they'll be silently rewritten.
+
+## Gotchas
+
+- `scripts/docker-secrets` must be **sourced** (`. $0`), not executed, when
+  loading secrets into the current shell. Executing it runs `exec "$@"` and
+  replaces the shell.
+- Integration tests require Vault KV **v1**. The `tests/setup` script
+  explicitly dismounts KV v2 and re-enables v1. If the CI Vault image
+  defaults change, tests will silently return empty values.
+- `make beta` and `make stable` commit directly to the current branch.

CLAUDE.md

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+@AGENTS.md