Add repository working guide for brand maintenance

[vitormattos]

Add a concise AGENTS.md as an operational entry point for humans and automated contributors maintaining the LibreSign brand repository.

The guide intentionally does not duplicate the brand manual or detailed architecture documentation. It points contributors to the canonical files and records the decision-making rules that are easy to lose when someone arrives fresh:

• source-of-truth hierarchy;
• where content, components, theme tokens, artwork, legal records, and licensing belong;
• how to handle normative brand changes;
• canonical-artwork and deterministic-generation rules;
• Typst maintenance and exact compiler-version policy;
• PDF accessibility expectations;
• immutable GitHub Action pins and Dependabot boundaries;
• licensing/trademark separation;
• common workflows for text, artwork, builds, and releases;
• DCO and review hygiene;
• stable-release gate after homologation.

The wording is written to be equally usable by a human maintainer or an automated contributor.

Commit includes:

Signed-off-by: Vitor Mattos <vitor@php.rio>