test: REUSE gate tests clean up on setUp failure and cover a subdirectory run

[futurebuildai]

Two follow-ups from the PR 87 review r3, neither blocking.

P3-A: temp dir cleanup runs even if setUp raises

Both test classes of .github/scripts/test_reuse_gate.py (InlineHeaderCheck and MainSubprocessCheck) used to call tempfile.mkdtemp in setUp and then _init_repo straight after, with the cleanup in tearDown. If _init_repo raised for any reason (for example git is missing on PATH), tearDown would not execute and the temp directory would leak on disk.

This change registers self.addCleanup(shutil.rmtree, self.tmp, ignore_errors=True) immediately after mkdtemp, before _init_repo is called. addCleanup runs even when setUp itself fails partway through, so the directory is gone regardless. shutil is imported at the top of the module now and the now-redundant tearDown blocks are dropped (their only job was the rmtree call).

P3-B: a subdirectory run is covered

MainSubprocessCheck now also has a test_main_from_subdirectory_still_finds_root_headerless_file case. It puts a headerless .go file at the throwaway repo's root and runs the gate with cwd set to a subdirectory (tmp/deep/nested). The assertion is exit 1 and that the summary names the file. Without a git-root scan in main() (for example if the lookup fell back to os.getcwd()), the empty subdirectory would produce zero missing files, the gate would print REUSE gate **passed**, exit 0, and this test would fail.

Mutation proof:

1. With the suite green, change reuse_gate.py main()'s git-root lookup from git rev-parse --show-toplevel to os.getcwd() and run:

$ python3 -m unittest discover -s .github/scripts -p test_reuse_gate.py
...
test_main_from_subdirectory_still_finds_root_headerless_file ... FAIL
======================================================================
FAIL: test_main_from_subdirectory_still_finds_root_headerless_file
----------------------------------------------------------------------
AssertionError: 0 != 1 : stdout=
## REUSE licensing gate

- reuse stub-0.0.0, spec stub
- Files scanned: **0**
- With copyright info: **0**
- With licensing info: **0**
- Licenses in use: none

REUSE gate **passed**.

stderr=

Ran 9 tests in 0.221s

FAILED (failures=1)

2. Restore the one file: git checkout -- .github/scripts/reuse_gate.py, rerun the suite, the same case is green again and the full 9-test run is OK:

Ran 9 tests in 0.217s

OK

Exit tests

• python3 -m unittest discover -s .github/scripts -p test_reuse_gate.py: 9 tests, OK
• python3 .github/scripts/reuse_gate.py against this checkout: REUSE gate **passed**., exit 0 (uses the installed reuse 6.2.0)

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