Correct SQL combinator README examples

[tim-smart]

Summary

• use SQL fragments for the mixed sql.and example
• align the combinator output comments with PostgreSQL placeholders
• add a patch changeset for @effect/sql

Validation

• pnpm lint-fix
• pnpm test run packages/sql-pg/test/Client.test.ts
• pnpm check
• pnpm build
• pnpm docgen

Closes EFF-206

[pullfrog]

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[changeset-bot]

🦋 Changeset detected

Latest commit: 253b663

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@effect/sql	Patch

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