fix: disallow relative paths in user-supplied cache dir

[stephantul]

This PR disallows relative paths as part of the user-supplied cache dir strategy. If the user supplies a relative path, we revert to the default index, and show a warning.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/cache.py	100.00% <100.00%> (ø)

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[greptile-apps]

Confidence Score: 4/5

Safe to merge; the change is narrow and correct, with the only open question being test completeness around the emitted warning.

The implementation is straightforward and handles all cases (unset env var, relative path, absolute path) correctly. The sole observation is that the new test does not assert the logger.warning call, so a future removal of that diagnostic would go undetected by the test suite.

tests/test_cache.py — the new test for the relative-path guard omits an assertion on the warning log.

_{Reviews (1): Last reviewed commit: "fix: disallow relative paths in user-sup..." | Re-trigger Greptile}

[Pringled]

Good catch