docs(readme): give JSON sidecar schema its own linkable section

[cameroncooke]

The JSON sidecar field table, default context keys, and custom-context notes were buried inside § "1. Export the snapshots from your test run", nested under the TEST_RUNNER_SNAPSHOTS_EXPORT_DIR env-var docs. The nearest shareable anchor was the env-var section, which is orthogonal to the data format.

This PR extracts that content into a dedicated ### JSON sidecar schema section (parallel to the existing ### Snapshot modifiers section) so it has its own stable anchor (#json-sidecar-schema) and can be linked to directly.

Changes:

• Moved the sidecar field table and context notes out of the env-var prose into a new ### JSON sidecar schema heading between ### Environment variables and ### 2. Upload to Sentry
• Added an inline link from the .json bullet in the env-var section to the new anchor
• No content was added or removed — only reorganised

Fixes EME-1191

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View Session in Sentry

[linear-code]

EME-1191