chore: module documentation standard (SPEC-005)

[kriss145]

Summary

• Adds SPEC-005 — a documentation standard for all @open-mercato/* packages in this monorepo
• Defines a three-level hierarchy: repo root README (module table) → packages/<name>/README.md (shortened docs with real content) → packages/<name>/docs/*.md (extended docs split into files)
• Introduces skill/SKILL.md as a required artifact for modules with extensible APIs (e.g. base classes, convention file patterns)
• CI enforcement is part of the implementation plan (Phase 1) — not included in this PR

What's in this PR

• .ai/specs/SPEC-005-2026-05-18-module-documentation-standard.md — full specification

Implementation plan (follow-up PRs)

• Phase 1: Update AGENTS.md, update scaffold-module skill to generate docs scaffolding, add GitHub Actions docs-check workflow
• Phase 2: Write documentation for carrier-inpost following the standard

Reference

The packages/pdf-generators module (on feat/pdf-generators) served as the design reference — its documentation structure is the closest existing example of what this standard formalizes.

[kriss145]

@pkarw @dominikpalatynski Could you look into it?