adopt-project.py: remove the temp work roots it makes without --work-dir, keep a recorded --source clone, and point the tests' TMPDIR at their basetemp #526

plan, check and execute each make a fresh openreposhape-adopt-XXXXXXXX directory in the system temp folder when --work-dir is not given (_work_root, adopt-project.py:6028-6032, called at :2088, :3543 and :5100) and nothing ever removes it: plan and check of a local source leave it empty, execute leaves both legs' and the split's working trees in it, and 2,992 of the first 3,000 such directories in one container's /tmp (15,206 in all) are empty. Direction: remove a work root that holds nothing when the command ends, and say where execute's stays.
— #457, body

plan --source <org/repo> without --work-dir clones into a temporary directory and records that path. If the directory is cleaned before check or execute, they refuse source-unclonable with text about the repository's name. Passing --source again recovers.
— #457, comment

The test suite never points TMPDIR at its own scratch for the adopt runs (tests/conftest.py sets none, and no adopt suite does; the five that set it are test_cli_reference, test_family_add_refusals and three scaffold suites), so every plan, check and execute a test runs without --work-dir leaves an openreposhape-adopt-* directory in the system temp folder, outside pytest's tmp_path retention: 15,206 sit in one container's /tmp (2,992 of the first 3,000 empty), 226,165 on the family pass's machine. Direction: one autouse fixture pointing TMPDIR into the session's basetemp; the tool-side half is the first line under #457.
— #461, body