Add LLM GitHub action for automated issue triaging and labelling

### Task

Update the label descriptions for the repo and implement an LLM-powered GitHub Action to automatically triage and apply relevant labels to newly created issues in the engine repository.

### Context

Navigating the engine repository and filtering for specific topics (such as sync issues) can be time-consuming without a consistent labelling system. However, manually triaging and labelling every new issue is tedious and prone to inconsistency.

To improve repository organization and make perusing issues easier for contributors, we can automate the initial triage process using an LLM. By feeding a predefined labelling guidance document to a Claude GitHub Action, newly opened issues can be automatically categorized. The automation should focus strictly on objective categorizations—such as issue type (`enhancement`, `bug`, `refactor`) and domain (`area: xyz`)—while avoiding subjective or lifecycle labels like `good first issue` or status indicators.

This is an experimental workflow to see if automated labels provide tangible value to maintainers and contributors; if it ends up being unused or unhelpful, the action can easily be removed later.

### Acceptance Criteria

* A guidance document defining the repository's issue labelling rules and categories is written and committed to the repo.
* A GitHub Action (using Claude?) is set up to trigger when new issues are created.
* The automation successfully reads label descriptions and applies appropriate type labels (e.g., `bug`, `enhancement`, `refactor`) and area labels (e.g., `area: xyz`) to new issues.
* The automation is explicitly configured to ignore subjective, lifecycle, or status labels (e.g., `good first issue`).

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Author: Bruno D
Discord: https://discord.com/channels/549281623154229250/1491032819063849081/1491154264020484106
Model: gemini-3.1-pro-preview

The issue names no existing files or tests; start by locating the repository's label descriptions and GitHub Actions configuration. Write the labelling guidance document and add an action triggered for newly created issues using the chosen LLM integration. Done means new issues receive only objective type and area labels, while lifecycle and status labels are excluded; verify with a test issue or workflow run.

Written by the indexing model from the issue text.