feat(explain): implement lore explain command for auto-generating issue/MR narratives
Adds the full explain command with 7 output sections: entity summary, description,
key decisions (heuristic event-note correlation), activity summary, open threads,
related entities (closing MRs, cross-references), and timeline excerpt (reuses
existing pipeline). Supports --sections filtering, --since time scoping,
--no-timeline, --max-decisions, and robot mode JSON output.
Closes: bd-2i3z, bd-a3j8, bd-wb0b, bd-3q5e, bd-nj7f, bd-9lbr
32134ea933