…est (#3371)

TestBuildMigratesLegacySessionsFromConfigSessionDir used t.Chdir(proj) to point Build at a temp project dir, which makes that dir the process cwd. On Windows a cwd directory cannot be removed, so t.TempDir cleanup failed (being used by another process) even though all assertions passed — this reddened the main-v2 Windows CI. Use the Build WorkspaceRoot option instead: it loads the project config without changing the process cwd. Fixes the #3247 regression.

… dirs (#3944)

The flat import (#3247) reconstructed v0.x sessions into the global
config session dir, but the desktop sidebar only lists the per-workspace
projects/<slug>/sessions dirs — so for desktop users the import
"succeeded" into a directory nothing reads, and upgraders kept reporting
their history gone (#3937).

- the per-workspace session-dir layout moves to config.ProjectSessionDir
  (the desktop helper now delegates), so the importer can route to it
- the importer reads each session's v0.x sidecar (<name>.meta.json):
  a live workspace routes the session into that workspace's dir, the
  summary becomes its display title; dead/missing workspaces fall back
  to the global dir as before
- a new one-time routed pass deliberately ignores the flat-import
  markers so existing upgraders get their stranded sessions re-homed;
  a global-dir file is only moved when its mtime matches the legacy
  event log (the flat import stamped it), so a same-named native v1+
  session is never touched

Trade-off: sessions a user deleted after the flat import reappear once
(in their workspace) during the routed pass; there is no record that
distinguishes "deleted by user" from "never imported", and recovering
every stranded upgrade is worth one resurrection.

Closes #3937

Co-authored-by: reasonix <reasonix@deepseek.com>