feat: [ENG-2858] add dream-scan/dream-finalize task types + brv dream CLI subcommands

Wires Phase 3 of the tool-mode dream pipeline:

- Two new daemon task types (dream-scan, dream-finalize) added to
  TaskExecuteSchema + TaskTypeSchema + TaskCreateRequest. Skipped in
  the provider-validation gate alongside curate-html-direct since they
  run pure deterministic code (no LLM).

- Two new daemon case handlers in agent-process.ts: dream-scan parses
  {kinds, scope, maxCandidates} and calls scanDreamCandidates with the
  shared searchKnowledgeService + runtimeSignalStore; dream-finalize
  parses {sessionId, archive[]} and calls finalizeDreamSession.

- Five new oclif subcommands under src/oclif/commands/dream/:
  - scan       — dispatches dream-scan, renders candidates per kind
  - finalize   — dispatches dream-finalize with archive list (or --archive-file)
  - sessions   — stateless v1 stub (returns empty list)
  - cancel     — stateless v1 stub
  - undo       — reuses existing dream-undo via buildUndoDeps