feat: add gateway issue dispatcher workflow (#2603)

Adds an agentic workflow that runs every 6 hours to audit open
`gateway`-labeled issues in `github/gh-aw` and create corresponding
tracking issues in `github/gh-aw-mcpg` with problem analysis and
proposed solutions.

## What it does

1. Lists all open issues in `github/gh-aw` with the `gateway` label
2. Skips issues that already have a linked `gh-aw-mcpg` tracking issue
(detected via comment URL)
3. For each unprocessed issue: analyzes the problem, researches the
codebase, and creates a tracking issue in `gh-aw-mcpg` with problem
description, analysis, proposed solution, and testing guidance
4. Comments on the original `gh-aw` issue with a link to the new
tracking issue

## Configuration

- **Schedule**: Every 6 hours (fuzzy)
- **Cross-repo access**: Uses `CROSS_REPO_PAT` secret for read/write
across both repos
- **Guard policy**: `min-integrity: none`, `allowed-repos:
[github/gh-aw, github/gh-aw-mcpg]`
- **Safe outputs**: `create-issue` → `github/gh-aw-mcpg`, `add-comment`
→ `github/gh-aw`

## Prerequisites

Add a `CROSS_REPO_PAT` repository secret with `issues:write` +
`repo:read` access to both `github/gh-aw` and `github/gh-aw-mcpg`.

Author: lpcox