fix: honor HOME in guest os.homedir

[NathanFlurry]

What changed

• Make guest node:os homedir() follow the effective process.env.HOME, including runtime mutations.
• Keep os.userInfo().homedir tied to the VM user identity (/home/agentos) so changing HOME does not customize passwd/user identity.
• Add a sidecar regression test comparing guest behavior with host Node for HOME=/home/foo and later process.env.HOME mutation.

Validation

• cargo test -p secure-exec-sidecar --test builtin_conformance os_homedir_follows_effective_home_env -- --nocapture

[railway-app]

🚅 Deployed to the secure-exec-pr-119 environment in rivet-frontend

Service	Status	Web	Updated (UTC)
secure-exec	😴 Sleeping (View Logs)		Jun 24, 2026 at 1:47 am

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🚅 Deployed to the secure-exec-pr-119 environment in secure-exec

Service	Status	Web	Updated (UTC)
secure-exec	😴 Sleeping (View Logs)	Web	Jun 24, 2026 at 1:45 am