hn_search_content now rejects unparseable dateRange.start/end strings
at the Zod input boundary via .refine(), instead of letting bad input
flow through to new Date(…).getTime() and producing NaN Algolia
numeric filters. Test covers the rejection path.

Field description tweaks across hn_search_content and hn_get_thread:
storyId now calls out the equals-id case for story hits; the empty-
results message reads more naturally; the depth guidance fixes a
commentId/itemId mix-up; and the comments[] description spells out
the BFS-by-rank order instead of leaning on the abbreviation.

CLAUDE.md version header synced from the stale 0.4.0 to 0.4.2.

Tests: 111 passing (up from 110).