fix(cli): make non-root read-only commands work on Linux (EPERM liveness, sbin PATH)

[pilat]

Two follow-up fixes to the Linux sudo work from #9 — both the kind of thing that only shows up the first time you actually drive it as a non-root user on a real Linux box, which is exactly where they surfaced.

The headline one: after an auto-elevated fleetbox up, a plain fleetbox ls or fleetbox ssh reported the running VM as stopped. The holder liveness probe used kill(pid, 0) == nil, but a non-root user probing the root-owned holder gets EPERM (the process exists, you're just not allowed to signal it across the uid boundary) — which the code read as "dead", so it never even reached the socket and fell through to "stopped". That quietly defeated the whole point of #9, where non-root read-only commands were supposed to just work. Now EPERM counts as alive; only a genuinely absent process (ESRCH) is "not running".

The second: the CLI's auto-elevation forwards the invoking user's PATH, and a stock Debian login PATH has no /sbin. The env wrapper overrides sudo's secure_path, so the elevated holder couldn't find iptables and network setup died — it only worked on #9's test box because that shell happened to have sbin on its PATH. The elevated PATH now always carries the sbin directories.

Both were validated on real hardware — the EPERM one on a non-root Linux box (the VM flipped from "stopped" to "running" with the fix), the PATH one by dogfooding the Linux path inside a fleetbox-booted VM. ADR-0023 and the architecture notes are updated to record both.

Checklist

• Changed the public API, package list, CLI surface, on-disk layout, or dependencies → ARCHITECTURE.md updated in this PR
• Made a new, hard-to-reverse design decision → added an ADR under docs/adr/ (next sequential number)
• Breaking change (! in the title) → the description spells out what callers must change

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⚙️ Run configuration

Configuration used: defaults

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Run ID: f865ef98-8088-419a-be7e-2f6e3e9fd5fe

📥 Commits

Reviewing files that changed from the base of the PR and between 1ad9b70 and 43a8580.

📒 Files selected for processing (7)

• ARCHITECTURE.md
• cmd/fleetbox/elevate.go
• cmd/fleetbox/elevate_linux.go
• cmd/fleetbox/elevate_test.go
• docs/adr/0023-linux-sudo-ux.md
• internal/control/control.go
• internal/control/liveness_test.go

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/linux-nonroot-status

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