docs: sync specs/index.md with idempotency.feature

[mroderick]

What

Sync docs/specs/index.md embedded Gherkin with the canonical docs/specs/idempotency.feature.

Changes

• +13 scenarios — RFC 9457 instance and retryable field validation across all error types (missing key, empty key, too-long key, commas, key reuse, fingerprint conflict, concurrent request, store unavailable)
• +2 scenarios — idempotency_key extension field for empty key and key reuse errors
• +3 scenarios — Infrastructure Errors (Store Unavailable → 503) section
• 3 renames — RFC 7807 → RFC 9457 in scenario names
• 1 overview update — RFC 7807 → RFC 9457 in description

Verification

# Extract embedded Gherkin and diff against canonical feature file
sed -n '/^```gherkin/,/^```$/p' docs/specs/index.md | sed '1d;$d' | diff - docs/specs/idempotency.feature
# Should show only trailing newline difference

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