Add FoodGraph live presentation deck

[teopaleface]

Outcome

Adds an editable seven-slide PowerPoint framing deck for the live FoodGraph demo and links it from the README and release handoff.

The deck covers the frozen evidence base, graph architecture, three demo guarantees, safe Text2Cypher, audit trail, four-person handoff, and close. It is intentionally a short framing artifact; the notebook remains the live demonstration.

Verification

• rendered every slide and inspected each at full size;
• slides_test.py passed with no overflow;
• visual QA corrected the initial table-position issue before this commit;
• tracked-file secret scan and diff check passed.

Closes #17