[PPTX] --pptx-only CLI flag + pipeline integration — auto-export .pptx at end of every run

[DogInfantry]

Part of epic: #32
Depends on: #44 (all 10 slides must be implemented first)

What

Two tasks in one issue since they are tightly coupled:

1. Add --pptx-only argparse flag to pipeline_v2.py
2. Auto-generate .pptx at end of every default pipeline run (controlled by config.yaml flag export_pptx: true)

CLI

# Full pipeline + auto .pptx export (default if export_pptx: true in config)
python pipeline_v2.py --ticker AAPL

# Skip HTML/PDF note, generate .pptx only
python pipeline_v2.py --ticker AAPL --pptx-only

# Disable pptx export for this run only
python pipeline_v2.py --ticker AAPL --no-pptx

config.yaml flag

export:
  html_note: true
  pdf_note: true
  pptx: true       # auto-generate .pptx after every full run
  tearsheet: true

Output path

outputs/{TICKER}_{DATE}_research_note.pptx

Acceptance Criteria

• --pptx-only flag exists in argparse
• --no-pptx flag disables export for single run
• export_pptx: true in config.yaml triggers auto-export after full run
• .pptx file saved to outputs/ with timestamped filename
• File opens without error in PowerPoint and LibreOffice
• File size under 5MB for standard 10-slide deck
• README updated with export flag documentation