cargo.build: a persistent build cache shared by all worlds

- The cargo target folder is <cache>/cargo-target, next to Wasmtime's
  cache, which moves to <cache>/wasmtime: Wasmtime's cleanup worker
  deletes unknown entries in its folder, also Cargo's artifacts.
- Builds use the SDK at <cache>/sdk/<content hash>/, so cargo reuses
  the compiled SDK in every world. /plugins/cargo/sdk/ stays for agents.
- Tests run in the release profile and share compiled crates with the
  release build.
- The cargo and cli tests use a persistent cache in CARGO_TARGET_TMPDIR:
  warm, the cargo suite takes 11 s (was 235 s) and the cli suite 6 s
  (was 149 s).
- nix flake check runs ignored stress tests. AGENTS.md: rules for fast
  tests.

README.md

@@ -17,8 +17,9 @@ an LLM (OpenAI Responses or Anthropic Messages API). No clone needed:
 nix run git+https://gitea.surma.technology/surma/sausage
 ```
 
-This creates a world in `./world` the first time (it builds the agent, which
-takes a few minutes), then starts it with a console. Later runs start the
+This creates a world in `./world` the first time (it builds the agent: the
+first build on a machine takes a few minutes, later worlds reuse the compiled
+crates), then starts it with a console. Later runs start the
 existing world. The two steps on their own:
 
 ```sh
@@ -32,6 +33,10 @@ the base URL, the model, and the token. It reads `SAUSAGE_API`,
 values go to `world/config/agent.json`. Both commands take another folder as
 an argument, for example `... -- run my-world`.
 
+Compiled code is cached outside the world, in `$XDG_CACHE_HOME/sausage/`
+(`~/.cache/sausage/` without the variable). All worlds share it.
+`--cache-dir <folder>` changes it for `init` and `run`.
+
 To keep a world on the newest version, run it in a loop:
 
 ```sh