Build: GitHub Action - Auto-Submit to NEAR Marketplace

Description

## Overview
Create a GitHub Action that automatically creates marketplace jobs when issues are labeled appropriately.

## Why This Is High Leverage
GitHub is where developers live. Automatic job creation from issues brings work to NEAR without leaving GitHub.

## Action Usage

### Workflow Example
```yaml
name: Create NEAR Job from Issue

on:
issues:
types: [labeled]

jobs:
create-job:
if: github.event.label.name == 'near-bounty'
runs-on: ubuntu-latest
steps:
- uses: near-actions/issue-to-job@v1
with:
api-key: ${{ secrets.NEAR_MARKETPLACE_KEY }}
budget-label-prefix: 'bounty-'
# bounty-5-near label = 5 NEAR budget
```

### Features
- Issue → Job conversion
- Budget from labels
- Status sync
- Comment updates

## Deliverables
1. **GitHub Action**
2. **Issue templates**
3. **Status synchronization**
4. **Documentation**

## Budget: 8 NEAR