`FirstRunDetector.detectExistingProjects()` walks ancestor directories
looking for `.fusion/` projects, stopping only at `homedir()` and the
filesystem root. On systems where a prior Fusion session has left a
`/tmp/.fusion/fusion.db` (e.g. running tests, ephemeral dashboards,
crashed processes), the walk crosses into `/tmp` and incorrectly
"discovers" that stale state as a project.
This was caught by tests in packages/core/src/__tests__/store.test.ts
that intermittently failed when a real Fusion session had been run
on the same box. The walk would find the test's tmp dir AND the host's
real `/tmp/.fusion/fusion.db`, polluting the project-detection invariants.
Fix: import `tmpdir` from `node:os` and add it as a third walk-stop
boundary alongside `homedir()` and `/`. The OS temp directory is a
shared system surface and should never itself host a project.
The bug affected any user with a stale `/tmp/.fusion/` from a previous
session — invisible most of the time, but caused FirstRunDetector to
misclassify project state.
Tested: 4 previously-flaky tests in packages/core now pass deterministically.
3796ba087c