refactor: generic skills build system with auto-discovery

[Rodriguespn]

Summary

This PR refactors the build system from a single-skill setup (postgres-best-practices-build) to a generic, extensible build system (skills-build) that automatically discovers and processes any skill in the skills/ directory.

Key changes:

• Renamed packages/postgres-best-practices-build → packages/skills-build
• Added auto-discovery: scans skills/ for subdirectories containing metadata.json
• Build/validate all skills at once or target a specific skill with -- {skill-name}
• Updated documentation (AGENTS.md, CONTRIBUTING.md) with new structure
• Zero configuration needed to add new skills

Motivation

Previously, the build system was tightly coupled to the postgres-best-practices skill. Adding a new skill would require modifying build configuration and hardcoded paths. This refactor enables:

1. Scalability - Add unlimited skills without touching the build system
2. Simplicity - Just create a directory with the right structure
3. Consistency - All skills follow the same build/validation process

New Repository Structure

agent-skills/
├── skills/
│   └── {skill-name}/
│       ├── metadata.json         # Required: skill metadata
│       ├── AGENTS.md             # Generated: compiled rules output
│       └── rules/
│           ├── _sections.md      # Required: section definitions
│           ├── _template.md      # Optional: rule template
│           └── {prefix}-{name}.md  # Rule files
│
└── packages/
    └── skills-build/             # Generic build system (renamed)

How to Add a New Skill

Adding a new skill is now configuration-free. Follow these steps:

1. Create the directory structure

mkdir -p skills/my-new-skill/rules

2. Create metadata.json

{
  "version": "1.0.0",
  "organization": "Your Org",
  "date": "January 2026",
  "abstract": "Brief description of what this skill teaches AI agents."
}

3. Create rules/_sections.md

Define categories for your rules with impact levels:

## 1. First Category (first)
**Impact:** HIGH
**Description:** What this category covers.

## 2. Second Category (second)
**Impact:** MEDIUM
**Description:** What this category covers.

The prefix in parentheses (e.g., first, second) maps to rule file prefixes.

4. Add rule files

Create rules in rules/ named {prefix}-{rule-name}.md:

---
title: Action-Oriented Title
impact: HIGH
impactDescription: Quantified benefit (e.g., "5-20x improvement")
tags: keyword1, keyword2
---

## Title

1-2 sentence explanation of the rule.

**Incorrect:**
\`\`\`sql
-- bad example
\`\`\`

**Correct:**
\`\`\`sql
-- good example
\`\`\`

5. Build

npm run build                    # Build all skills
npm run build -- my-new-skill    # Build only your skill