Story 6-4/6-5: Chat with PDF (done) + PPTX watermark PLG

Story 6-4 — Chat with PDF:
- Feature déjà implémentée (document-qa-overlay.tsx, ingestion, search)
- Marquée done dans sprint-status

Story 6-5 — PPTX Export Watermark (PLG viral loop):
- lib/brainstorm/export-pptx.ts: addWatermark + withWatermark helpers
- lib/ai/tools/pptx.tool.ts: même pattern monkey-patch addSlide
- Watermark 'memento-note.com' 7pt gris bas-droite sur chaque slide
- Zéro modification des 14+ fonctions de slides existantes
- 174 tests passent, aucune erreur TS

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/sprint-status.yaml

@@ -66,8 +66,8 @@ development_status:
   6-1-onboarding-activation: done             # story-onboarding-activation.md
   6-2-markdown-roundtrip: review        # brief-markdown-roundtrip.md
   6-3-brainstorm-canvas-finalize: review        # story: 6-3-brainstorm-canvas-finalize.md
-  6-4-chat-with-pdf: backlog
-  6-5-pptx-export-watermark: backlog
+  6-4-chat-with-pdf: done                   # already implemented: document-qa-overlay.tsx + document-ingestion + document-search tool
+  6-5-pptx-export-watermark: done              # story: 6-5-pptx-export-watermark.md
   epic-6-retrospective: optional