feat: add pg-ast package (copy of @pgsql/utils without runtime schema)

[pyramation]

Summary

Creates a new pg-ast npm package as a lightweight copy of @pgsql/utils that excludes the runtime schema functionality. This package provides PostgreSQL AST helper functions for building AST nodes programmatically.

Key changes:

• Copied @pgsql/utils package structure to packages/pg-ast/
• Disabled runtime schema generation in scripts/pg-proto-parser.ts (runtimeSchema.enabled: false)
• Removed runtime-schema.ts from the source files
• Updated README with note directing users to @pgsql/utils if they need runtime schema
• Updated package.json with name "pg-ast" and appropriate description
• Cleared CHANGELOG.md for the new package

Review & Testing Checklist for Human

• Verify the package name "pg-ast" is correct (not scoped like @pgsql/utils) - this was the user's intent since they own the npm name
• Confirm the tests in __test__/utils.test.ts pass and don't depend on runtime-schema functionality
• Check that the README note about using @pgsql/utils for runtime schema is clear and accurate
• Verify CI passes - the build:proto script may fail since it depends on workspace packages, but the source files are pre-generated

Test plan: Run pnpm install && cd packages/pg-ast && npx tsc --noEmit to verify TypeScript compiles. Run tests with pnpm test in the pg-ast directory.

Notes

The source files (asts.ts, wrapped.ts) are auto-generated and were copied from @pgsql/utils. The build:proto script won't work standalone since pg-proto-parser is a workspace dependency, but this shouldn't be an issue since the generated files are already in place.

Link to Devin run: https://app.devin.ai/sessions/cc86c0f40d274912b1531258fdcdbdf1
Requested by: Dan Lynch (@pyramation)

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