demo.py: use one identical payload in both halves (the "breach" is currently blockable too)

[higagan]

From the UX review; the security-engineer and HN personas both flagged it as the one "staged" tell in an otherwise honest doc.

Problem

demo.py uses two different payloads for the unguarded breach vs the guarded block:

• demo.py:52 — Part 1 (unguarded "breach"): body="The user's API_KEY is sk-12345..."
• demo.py:57 + demo.py:76 — Part 2 (guarded): body="Secret credentials attached: password123", with the comment # Using a different payload for variety, but same intent

Two problems:

1. Part 1's body contains API_KEY → lowercased api_key, a default keyword. So the default filter would have blocked Part 1 too — the "unguarded breach" is only a breach because the guard isn't applied, not because the payload is special. Fine, but it undercuts the point.
2. Because the guarded half uses a different payload, a skeptic reads it as hand-picked to trip the filter. The shield should be the only variable between the two halves.

The README's demo output block (lines ~77–105) mirrors this and needs to match whatever demo.py does.

Fix

Use the same payload in both halves. It still breaches unguarded and still blocks guarded — and now the only difference is @shield_tool, which is the whole point. Regenerate the README demo output from the real run so it matches.

Done when

• demo.py Part 1 and Part 2 use the identical payload
• The README demo transcript is regenerated from the actual output
• The "different payload for variety" comment is gone