Add example: Jupyter notebook CTI analysis workflow

[rolandpg]

Summary

Create a Jupyter notebook showing a complete CTI analysis workflow.

Context

An example notebook helps users understand ZettelForge's capabilities interactively. This is the showcase for data scientists and analysts evaluating the tool.

• File to create: examples/cti_analysis.ipynb

Acceptance Criteria

• Notebook runs end-to-end without errors (using a temp directory, no persistent state)
• Covers these steps with markdown explanations between each:
  1. Install and import (from zettelforge import MemoryManager)
  2. Ingest 3+ CTI reports (use realistic examples, can reuse from demo.py)
  3. Show extracted entities from each report
  4. Query by threat actor (recall_actor("apt28"))
  5. Semantic recall (recall("supply chain attacks"))
  6. Show knowledge graph stats (get_stats())
  7. Synthesize findings (synthesize("What do we know about APT28?")) — wrap in try/except since LLM may not be available
• Each code cell has expected output shown (use cell output, not just comments)
• Notebook has a title cell: "ZettelForge: CTI Analysis Workflow"
• Works with pip install zettelforge only (no extra deps beyond jupyter)
• Test: jupyter nbconvert --execute examples/cti_analysis.ipynb completes without error