feat(tools): add microsandbox terminal backend

Adds microsandbox (https://microsandbox.dev) as a terminal backend alongside
docker/singularity/modal/daytona/ssh. Commands run inside a libkrun microVM
with its own kernel — meaningfully stronger isolation than the shared-kernel
container backends, without a cloud dependency or a daemon.

Backend shape is a thin wrapper around the msb CLI: one long-lived sandbox
per environment (msb create), command execution via msb exec, teardown via
msb stop + msb remove. Env-var filtering mirrors the Docker backend —
explicit docker-style microsandbox_forward_env / microsandbox_env lists,
skill passthroughs still filtered through _HERMES_PROVIDER_ENV_BLOCKLIST.

Files:
- tools/environments/microsandbox.py — new MicrosandboxEnvironment backend
- tools/terminal_tool.py — dispatch, container_config keys, image resolution
- hermes_cli/config.py — default microsandbox_* entries + env var sync
- cli-config.yaml.example — 'Option 7' documented config block
- tests/integration/test_microsandbox_terminal.py — skip-if-no-KVM
  integration tests for basic exec, filesystem, isolation, and the
  secret-leak regression

Host requirements: Linux with /dev/kvm readable (or macOS on Apple Silicon)
and msb on PATH or at MSB_PATH. Install: curl -fsSL https://install.microsandbox.dev | sh

Follow-up PR will wire this into the hermes_cli/setup.py wizard.

cli-config.yaml.example

@@ -226,8 +226,32 @@ terminal:
 #   daytona_image: "nikolaik/python-nodejs:python3.11-nodejs20"
 #   container_disk: 10240          # Daytona max is 10GB per sandbox
 
+# -----------------------------------------------------------------------------
+# OPTION 7: Microsandbox libkrun microVMs (local)
+# Commands run in a libkrun microVM with its own kernel.
+# Great for: strong isolation from the agent's env and filesystem while keeping
+# everything local (no cloud dependency).
+# Requires: Linux with /dev/kvm (or macOS on Apple Silicon), `msb` on PATH or
+# MSB_PATH set. Install: curl -fsSL https://install.microsandbox.dev | sh
+# -----------------------------------------------------------------------------
+# terminal:
+#   backend: "microsandbox"
+#   cwd: "/root"                    # Path inside the VM
+#   timeout: 180
+#   lifetime_seconds: 300
+#   microsandbox_image: "python:3.12"  # Any OCI image usable by msb
+#   # Optional: explicitly forward selected env vars into the VM.
+#   # Values come from your current shell first, then ~/.hermes/.env.
+#   # Warning: anything forwarded here is visible to commands in the VM.
+#   # microsandbox_forward_env:
+#   #   - "GITHUB_TOKEN"
+#   # microsandbox_env:
+#   #   PYTHONUNBUFFERED: "1"
+#   # microsandbox_volumes:         # SOURCE:GUEST_PATH, host path or named volume
+#   #   - "/host/data:/mnt/data"
+
 #
-# --- Container resource limits (docker, singularity, modal, daytona -- ignored for local/ssh) ---
+# --- Container resource limits (docker, singularity, modal, daytona, microsandbox -- ignored for local/ssh) ---
 # These settings apply to all container backends. They control the resources
 # allocated to the sandbox and whether its filesystem persists across sessions.
   container_cpu: 1              # CPU cores