tests: build a real unit-test net for the deterministic cores (CODE_HEALTH #1) #38

CODE_HEALTH #1 (see docs/CODE_HEALTH.md in #37 for full context).

tests/ contains only test_smoke.py — 3 assertions against ~40K lines of source. Every package (query/, agent/, service/, graph/, executor/, retrieval/, scoring/, …) has zero tests. pytest + pytest-asyncio + CI wiring already exist; only the tests are missing.

Why now: every other refactor in CODE_HEALTH is unsafe without a regression net, and regressions currently surface only in expensive benchmark runs.

Scope — unit tests for the LLM-free deterministic cores first:

• graph/store.py (add/get/edges/persistence round-trip)
• executor/runner.py (plan validation, atomic execute, rollback, orphan reconnection)
• scoring/ranker.py (PageRank + recency weighting)
• retrieval/bm25.py (indexing, scoring, determinism)
• query/text_utils.py (tokenize/stopwords)
• config.py (env parsing, YAML load)

Rule going forward (now in CLAUDE.md): refactors land with tests for the code they touch.