fix(security): move the interview protection note to the rule that provides it (#337)

The two-line comment above `documents/interview/**` says interview prep and
experience records live there. Nothing has ever written to that directory:
/interview saves its pack to
documents/applications/<company>_<role>/interview_prep_<stage>.md, covered by
the documents/applications/** rule. `git grep documents/interview` returns
only the two declarations of the rule itself (.gitignore and
REQUIRED_IGNORE_RULES), `git log --all -- 'documents/interview*'` is empty,
and documents/README.md documents the applications path outright.

Nothing leaks - the comment is the defect, and it is the misleading kind. It
is the one dedicated, well-argued line about interview material in the
personal-data block, so an auditor checking that the framework's most
sensitive artifact is covered reads it and stops, at the only path in the
block with no writer.

The comment's description of what needs protecting was always right; only its
location was wrong. It now sits above documents/applications/**, the rule that
actually provides that protection, so a reader auditing the block finds the
reasoning attached to the rule doing the work. documents/interview/** stays -
REQUIRED_IGNORE_RULES pins it, so dropping it from .gitignore alone turns CI
red, and it is harmless defence in depth - relabelled in both files as
belt-and-braces rather than the primary guard.

The new check-ignore case in GitignorePatternBehaviorTests derives the
prep-pack path from /interview's own spec instead of hardcoding it. That
distinction is the whole value of the test: a hardcoded path pins only that
documents/applications/** still matches that shape, which security_guards.py
already catches first, and stays green if /interview moves its output -
leaving the corrected comment stale exactly the way this issue found it.
Since #329 the spec states the location in two pieces - Step 1 derives the
archive folder, Step 3 names interview_prep_<stage>.md - so the test pins both
fragments separately and composes the concrete path from them. Mutation-
verified on each half: repointing the folder at documents/prep_packs/, and
renaming the file, both fail this test while `python3 tools/security_guards.py`
still reports OK.

The class's temp-repo setup moved to setUp for the second case.

ayobamiseun reviewed the pre-rebase branch and called all three rebase hazards
in advance: the split literal, the released CHANGELOG context, and the setUp
re-merge. Reached independently here during the rebase; the review was posted
first.

tools/security_guards.py

@@ -80,6 +80,8 @@ REQUIRED_IGNORE_RULES = [
     "documents/references/**",
     "documents/applications/**",
     "documents/postings/**",
+    # Belt-and-braces, not the primary guard: nothing writes here.
+    # /interview's prep packs land under documents/applications/**, above.
     "documents/interview/**",
     "job_search_tracker.csv",
     "gmail_sync/",