chore: remove pg-ast package from pgsql-parser

[pyramation]

pg-ast is now maintained and published from constructive-io/constructive (pg-ast@2.20.0 is the latest dist-tag on npm). The pgsql-parser repo's packages/pg-ast is a stale, duplicate copy (17.9.12) that is not consumed by any other package in this monorepo.

What changed:

• Delete packages/pg-ast.
• Update pnpm-lock.yaml to remove the packages/pg-ast importer.
• Remove pg-ast from AGENTS.md and .agents/skills/code-generation/SKILL.md.

After this, pg-ast will be released from constructive (bumped to 17.10.0) and pgsql-parser can consume it from npm if needed.

Link to Devin session: https://app.devin.ai/sessions/71d24cab0da24e7b83d3f5a6e9fef9a2
Requested by: @pyramation

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