Step 6 of v2.0 plan. SessionStart hook fires on source: resume or
source: compact, walks up to 3 levels searching for
NEXT-SESSION-*.local.md, injects content via additionalContext, and
archives the file (rename to *.archived.local.md) to prevent stale-load
in later sessions. 9 tests cover sources, multi-level search,
topic-slug variants, archive filtering, malformed payload.
