v0.18.6

Added

• Watcher-seeded idea specs now preserve the original idea markdown under
  millrace-agents/intake/ideas/<root_idea_id>.md and reference that
  runtime-owned artifact before transient ideas/inbox/ source files.

Fixed

• Closure-target creation now prefers the durable idea source artifact before
  legacy references, so Arbiter setup no longer depends on an inbox markdown
  file remaining in place after watcher intake.
• Backlog-drain closure recovery now marks Planning blocked with
  missing_root_idea_source and emits root_idea_source_missing when every
  source candidate is unavailable, instead of terminating the daemon loop.