Move the "explicit scratch means no project" decision into the one resolver
every surface funnels through, `kanban_db.create_task`: board-project
inheritance now runs only when the caller left `workspace_kind` open
(`None`), and `workspace_kind` defaults to scratch after that check. The
tool handler keeps the #106347 fix for `project=""` (no `or` collapse) and
`board=` scoping but drops its handler-local sentinel logic, since the
resolver now owns the rule; the `self_task` project inheritance for
dispatcher-owned workers is unchanged.
Sibling surfaces had the same bug through the same line and are fixed by
the same change:
- CLI `hermes kanban create --workspace scratch` on a project-scoped board
produced a project worktree; `--workspace` no longer defaults in
argparse so the resolver can tell "omitted" from "scratch".
- Dashboard `POST /tasks` with `workspace_kind: "scratch"` did the same;
`CreateTaskBody.workspace_kind` defaults to `None` for the same reason.
- `kanban_swarm.create_swarm` threads `None` through for consistency.
Tests: one resolver invariant in test_kanban_board_project.py (explicit
scratch stays scratch, omitted still inherits) and the salvaged tool test
folded into a single parametrized matrix over scoped/unscoped target boards.
b7bef04861