Fix fragile bare imports (concept_extractor → relative)

[StigNorland]

context.py:26 and concept_extractor.py:25 use bare from concept_extractor import ... instead of relative .concept_extractor. These only resolve if src/library is on sys.path, inconsistent with the .graph-style relative imports elsewhere.

• Switch to relative imports (or make the package layout explicit and consistent)

Refs: src/library/context.py:26, src/library/concept_extractor.py:25

[StigNorland]

Finding while investigating: the bare from concept_extractor import extract_concepts is guarded by try/except, so it does not break the app at runtime — it silently sets _EXTRACTOR_AVAILABLE=False. Worse, src/library/concept_extractor.py turns out to be a stale duplicate of an older context.py (it contains Turn/BackingStore/ContextAssembler, not an extractor); the imported extract_concepts function exists nowhere. So this is not an import-style fix — it's dead code. Folding the real cleanup into #17. Demoting #11 from blocker.