Delete eight wedev-era L1 packages that the M3.5 stack no longer
depends on: sandbox, tools, pathscan, initrunner, portproxy,
netcheck, container, constants. All were isolated dead code — no
import chain reached them from workspace/adapter/registry/reference/
template/command/lbderr/git/httpfetch.

The wedev "labdev owns one directory with tools/, home/, apps/"
sandbox model is incompatible with the Mise toolchain backend
landing in M4. Retiring now (per the no-legacy rule) avoids a
dual-shape window where Mise has to coexist with a competing
sandbox tools resolver.

Add internal/workspace/FindRoot(startDir) — the new primitive M4+
commands use to locate the active workspace by walking up from CWD
to the nearest labdev.yaml. Introduce workspace.FileName const so
the loader and FindRoot share one source of truth.

Drop the unused --root CLI flag and refresh cmd/root.go's Long help
+ README to the workspace-centric model (labdev.yaml + mise.toml +
adapters from labs/adapters + templates from labs/templates).

Binary still builds clean (9.3 MB). vet + all tests green.