docs(readme): cover Load Testing in the Capability Registry section

[sourabhd-cbu]

Why

The README's Capability Registry section only mentions Test Management, and the README has no Load Testing usage examples. With #458 shipping the Load Testing capability index on main, Load Testing is reached through the same five registry tools.

What

• Heading → "Capability Registry (Test Management & Load Testing)"; intro now covers 198 Test Management endpoints and 21 Load Testing capabilities, with one bullet per product on when to use the registry.
• "Writes ask first" note mentions starting/stopping load test runs.
• A Load Testing prompt example under each of the five tools (listProducts, describeEntity, searchCapability, describeCapability, invokeCapability).
• New "⚡ Load Testing" usage-examples section alongside Test Management (find, run, status, results, compare, quota).

README only — no code changes.

Depends on

#458 (Load Testing capability index on main). Merge after, or together with, #458 so the README doesn't describe tools that aren't in the release yet.

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