Start a from-scratch rewrite of agent-vm on top of microsandbox. The original
Bash/Python tree is removed on this branch and continues to live on main until
the rewrite reaches v1.
This phase only sets up the build:
- PLAN.md and ARCHITECTURE.md document the phased roadmap and the
design-decision log; ARCHITECTURE grows after each phase.
- microsandbox added as a git submodule at vendor/microsandbox so the Phase 3
SecretValue::File extension can branch the fork in place.
- Cargo workspace at the root with crates/agent-vm/ as the binary crate.
- Hello-world main.rs that boots an alpine sandbox via the microsandbox SDK
to prove the wiring is correct end-to-end (cargo check passes).
