fix(start-os): use a private temp file for the login banner's db snapshot

[JesseMarkowitz]

Problem

projects/start-os/build/lib/motd staged its start-cli db dump output at a fixed path:

DB_DUMP="/tmp/startos_db.json"

pam_motd runs the script as root at login, so that file is created root-owned. Every later non-root invocation then fails to write it, and the banner silently degrades to Services: Unknown, WAN: N/A, NTP: Unknown — the exact fields parse_essential_db_info exists to fill.

Reproduced on a 0.4.0.1 box as the start9 user:

start9@orion:~$ /usr/lib/startos/motd
/usr/lib/startos/motd: 7: cannot create /tmp/startos_db.json: Permission denied
    │ Services: Unknown                WAN:     N/A                     │
    │ Local:    192.168.50.204         NTP:     Unknown                 │

It is also a predictable path in a world-writable directory.

Fix

Use mktemp, and remove the file on every exit path. The previous code also leaked the snapshot when start-cli was absent or the dump failed — both of those returned without cleanup.

Verification

Ran the script against a live 0.4.0.1 server with the blocking /tmp/startos_db.json still in place:

• Before: Services: Unknown, WAN: N/A, NTP: Unknown
• After: Services: 5/5 running, WAN: 65.78.82.12, NTP: Synced

Also exercised the two failure paths — server unreachable, and start-cli not on PATH. Both degrade to the same banner as before and leave zero temp files behind. sh -n clean.

Two things for a maintainer to confirm

1. Base branch. CONTRIBUTING.md says to target a next/* branch when the latest release is not a pre-release, and start-os/v0.4.0.1 is tagged. But next/patch still has the pre-monorepo layout (no projects/), so this is based on master instead. Happy to retarget.

2. Changelog heading. ## [0.4.0.1] is a cut origin tag, so per the root AGENTS.md this entry goes under a new heading — ## [0.4.0.2]. I did not bump the manifests that heading is coupled to, because per VERSION_BUMP.md an OS revision bump also requires a new version/v0_4_0_2.rs migration module and a Current change, which is a release action rather than part of a bug fix. Say the word if you'd rather this land under a different heading.

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