feat(nodeenv): clean "src" directory by default

[ekalinin]

Closes #205.

Problem

A prebuilt install leaves a full copy of node under <env>/src, so an environment is twice the size it needs to be. Measured on macOS arm64 with node 26.9.0:

nodeenv --prebuilt ne1              -> 441M total, src/ = 221M
nodeenv --prebuilt --clean-src ne1c -> 221M total

Nothing reads that copy back. The "already downloaded" check looks for src/node-v<ver>, while a prebuilt archive unpacks into src/node-v<ver>-<platform>, so in the default mode the directory is not even a download cache - the tarball is downloaded again on every run.

Two smaller facts from the same path: clean_src was not a Config attribute, so it could not be set in a config file at all, and -n system created src unconditionally and left it empty.

Change

--clean-src is now the default, and --no-clean-src opts out.

Keeping src is worth it in exactly one case: with --source the directory holds the source tree that a repeated --force build reuses. That case now needs --no-clean-src, which is noted in CHANGES. A default that differs per install mode would be invisible from --help, so both modes clean by default.

--clean-src and -c keep working. pre-commit passes the flag explicitly (['-mnodeenv', '--prebuilt', '--clean-src', envdir]), and a test asserts it still parses.

clean_src also becomes a Config attribute, so it can be set in tox.ini, setup.cfg or ~/.nodeenvrc:

[nodeenv]
clean_src = False

mkdir(src_dir) stays unconditional: the empty src from -n system is now removed by the default cleanup, and it survives only under an explicit --no-clean-src, which is what that flag asks for.

Verified

command	src after the run
nodeenv --prebuilt ne1	removed, 221M, node and npm work
nodeenv --prebuilt --no-clean-src ne2	kept, 441M
nodeenv -n system ne3	removed (used to be an empty directory)
clean_src = false in a config file	kept
nodeenv -n system -c ne5	removed, flag still accepted

235 tests pass (233 unit plus the 2 integration smoke tests), flake8 --extend-ignore=E127 nodeenv.py tests setup.py is clean.

Out of scope

A shared download cache outside the environment, which would fix the repeated downloads that prebuilt mode does today. That also makes the dead node_src_dir check meaningful again, and both belong together in a separate change.