The helper under test writes to os.environ directly, bypassing
monkeypatch tracking. Without an explicit snapshot/restore fixture,
the mutation leaks into subsequent tests and breaks TestSharedBoardPaths
(kanban path resolution reads HERMES_KANBAN_BOARD and routes through
boards/<leaked-slug>/ instead of the test's own HERMES_HOME).
Add an autouse fixture that snapshots the env var before the test and
restores (or pops) it after, regardless of what the helper did.
Without an explicit pin, in-process kanban tools and shelled-out
`hermes kanban …` subprocesses resolve the active board on different
paths: the env var when set, otherwise the global `<root>/kanban/current`
file. When a concurrent session toggles the current-board pointer
mid-turn, the same chat ends up routing tool calls to board A while its
shell calls hit board B, surfacing as phantom "no such task" errors.
Pin the resolved board into env once at `cmd_chat` boot when
HERMES_KANBAN_BOARD isn't already set. Mirrors what the dispatcher does
for spawned workers (kanban_db.py:2622-2623). Idempotent and a no-op
when the env is already pinned by the caller.
Closes#20074
