feat: [ENG-2858] add 4 tool-mode dream candidate generators (link, merge, prune, synthesize)

Four new modules under src/server/infra/dream/tool-mode/ that produce
deterministic candidate sets for the calling agent to reason over:

- link-candidates.ts (9 tests): BM25 pairs above 0.5, exclude already-linked
- merge-candidates.ts (9 tests): BM25 pairs above 0.85, near-duplicate detection
- prune-candidates.ts (11 tests): importance < 35 OR mtime past tier threshold,
  skip core, sorted stalest-first
- synthesize-candidates.ts (9 tests): domain overviews + existing synthesis
  list, different shape per design

No LLM calls anywhere. Existing operations/* files untouched. Per-topic
searches run in parallel via Promise.all.