feat: [ENG-2858] add tool-mode dream topic loader + session orchestrator

Two new modules under src/server/infra/dream/tool-mode/:

- topic-loader.ts (9 tests): walks .brv/context-tree/ recursively, parses
  <bv-topic> attrs (title, summary, related), captures mtime, joins with
  sidecar signals. Best-effort: malformed files skipped, hidden dirs
  ignored.

- dream-session.ts (10 tests): scanDreamCandidates loads topics + runs all
  4 candidate generators in parallel, returns a unified envelope keyed by
  kind. finalizeDreamSession moves loser topics from .brv/context-tree/
  to .brv/archive/ preserving directory structure and drops their sidecar
  entries. Both are stateless on the daemon side for v1 (session id is
  opaque).