putPage's INSERT used COALESCE(<chunkerVersion>, 1), so callers that don't
supply chunker_version (no MCP/subagent caller does — it's internal metadata)
landed new pages at version 1. Dream subagents write through putPage directly,
so their pages got v1 and doctor's contextual_retrieval_coverage check flagged
them as "older chunker_version" forever, even though they were chunked and
embedded with the current chunker.
Default the INSERT to MARKDOWN_CHUNKER_VERSION on both engines. The ON CONFLICT
UPDATE still COALESCE-preserves an explicitly supplied version. Add an
engine-level regression test.
