v3: host crate skeleton, userland workspace, AGENTS.md for v3

The workspace is now host/ and vfs/. v2 crates leave the workspace as reference.
host/src/exec.rs is the contract between the process table and the harness.

host/src/process/mod.rs

@@ -0,0 +1,8 @@
+//! The process table: WASI programs in their own stores, the process interface (spawn, wait, kill),
+//! `/proc`, and the `Exec` implementation. Owned by task T8.
+use std::path::Path;
+
+/// Run a shell in the world at `world`, or one command with `command`. Returns the exit status.
+pub async fn sh(_world: &Path, _command: Option<String>) -> anyhow::Result<i32> {
+    anyhow::bail!("the process table does not exist yet (T8)")
+}