feat: add .agents/skills/ directory + fix codegen output path to .agents/skills/

[pyramation]

Summary

Two changes to improve agent skill auto-discovery in the Constructive monorepo:

1. Fix codegen default output path — resolveSkillsOutputDir() in graphql/codegen/src/core/generate.ts now defaults to .agents/skills/ instead of skills/. This aligns codegen-generated skills with the .agents/skills/<name>/SKILL.md convention used by Devin, Claude Code, Cursor, and Copilot for auto-discovery.

2. Add .agents/skills/constructive-monorepo-setup/SKILL.md — A lightweight setup skill that cross-references public skills (pgpm, constructive-graphql, constructive-testing, etc.) without duplicating content. Provides quick-start commands for local dev.

Note: The existing skills/ directory (14 codegen-generated skill dirs) is left in place — it will be superseded on the next codegen run, which will write to .agents/skills/ instead.

Review & Testing Checklist for Human

• Breaking change for downstream projects: Any project using @constructive-io/graphql-codegen without an explicit skillsPath will now output skills to .agents/skills/ instead of skills/. Verify this is acceptable for all consumers, or consider whether skillsPath overrides are needed in existing codegen configs.
• Stale skills/ directory: This PR does not remove or migrate the existing skills/ directory (14 dirs: cli-admin, hooks-public, orm-objects, etc.). Decide whether to clean it up in this PR or leave it for the next codegen run.
• Verify codegen consumers: Run cnc codegen (or equivalent) against a live endpoint after merging to confirm skills are written to .agents/skills/ correctly.

Notes

• The codegen change is a single string literal swap ('skills' → '.agents/skills'). No type, import, or logic changes.
• The skillsPath config option still works as an override — only the default changes.
• Companion PRs exist in constructive-skills (Feat/use postgis pgvector #67, test pgvector postgis #68), constructive-private-skills (node-gyp build error on npm install #25), constructive-db (chore: remove apiExtensions and database_extension references #654), and constructive-sdk-agentic-flow (setup demo script #17) for the same .agents/skills/ migration.

Link to Devin session: https://app.devin.ai/sessions/d0a20ee2e5194f4e8c342c6bc12fea07
Requested by: @pyramation

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring