Add Lima sandbox environment for parallel feature testing

Implement a complete sandbox system for testing multiple features in parallel
using git worktrees and Lima VMs. Each worktree gets its own isolated VM with
Docker, full service stack, and unique IP via vzNAT networking.

- contrib/lima/provision.sh: Idempotent provisioning script (Docker, Go 1.26.1, Node.js 24, npm 11.8.0, Go tools, mkcert)
- contrib/lima/probo.yaml: Lima VM template with vz vmType, Rosetta, vzNAT, virtiofs mount
- contrib/lima/sandbox.sh: Lifecycle CLI (create, start, stop, restart, delete, ssh, exec, status, list)
- contrib/lima/README.md: Human documentation with prerequisites, quickstart, troubleshooting
- contrib/claude/sandbox.md: Agent reference doc for sandbox usage patterns
- GNUmakefile: Convenience targets for sandbox.sh commands
- AGENTS.md: Updated reference documentation index

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

AGENTS.md

@@ -33,6 +33,7 @@ Detailed guides for specific subsystems live in `contrib/claude/`:
 - [`contrib/claude/go-testing.md`](contrib/claude/go-testing.md) — Go test conventions (parallel, require vs assert, naming)
 - [`contrib/claude/go-worker.md`](contrib/claude/go-worker.md) — Go worker pattern (poll-based, bounded concurrency, FOR UPDATE SKIP LOCKED)
 - [`contrib/claude/go-service.md`](contrib/claude/go-service.md) — Go service orchestration (Run, graceful shutdown, crash propagation)
+- [`contrib/claude/sandbox.md`](contrib/claude/sandbox.md) — Lima sandbox environments (create, manage, access services)
 
 ## API Surface Rules