fix: env vars in exec and systemd mode

[sjmiller609]

Note

Medium Risk
Touches VM init and systemd service injection logic plus adds new integration tests that pull and boot real images; failures could affect guest boot/exec behavior and CI stability.

Overview
Fixes environment variable propagation inside guests so hypeman exec and the guest-agent see the instance env in both exec and systemd boot modes.

In exec mode, the guest-agent is now started with the same env as the entrypoint; in systemd mode the init code writes env vars to /etc/hypeman/env (shell-escaped) and wires it into hypeman-agent.service via EnvironmentFile. Adds unit tests for env-file quoting plus new end-to-end tests (Cloud Hypervisor + QEMU) that validate env access via exec and that entrypoints actually consume env (using bitnami/redis auth). Also updates .air.toml to run make build-embedded before rebuilding, and adds al.essio.dev/pkg/shellescape dependency.

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