Add agentic workflow for daily GPL dependency detection (#902)

Implements automated daily scanning of go.mod dependencies to detect
GPL-licensed packages (GPL, AGPL, LGPL) that may conflict with project
licensing requirements.

## Workflow Capabilities

- **Multi-source license detection**: Queries pkg.go.dev API, fetches
LICENSE files via GitHub MCP, and pattern-matches SPDX identifiers
- **Transitive dependency tracking**: Uses `go mod why` to trace GPL
packages through the dependency graph
- **Stateful operation**: Cache memory prevents duplicate issue creation
and tracks resolutions across runs
- **Actionable remediation**: Generated issues include dependency paths,
license implications, alternative packages, and removal commands

## Configuration

```yaml
schedule: daily                    # Fuzzy scheduling, auto-scattered
permissions: read-all              # Safe outputs for write operations
network: [proxy.golang.org, ...]  # Go ecosystem + GitHub API
safe-outputs:
  create-issue: max 3              # New GPL dependencies
  add-comment: max 2               # Resolution tracking
  noop:                            # Silent success
```

## Detection Scope

Identifies all GPL-family licenses requiring review:
- GPL-2.0/3.0 (all variants: only, or-later)
- AGPL-3.0 (network use = distribution)
- LGPL-2.0/2.1/3.0 (linking restrictions)

Issues include full dependency path, license type, removal strategies,
and verification commands. Comments on existing issues when GPL packages
are removed.

## Files

- `.github/workflows/gpl-dependency-checker.md` - Workflow definition
(editable without recompilation)
- `.github/workflows/gpl-dependency-checker.lock.yml` - Compiled GitHub
Actions workflow

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/github/gh-aw-mcpg/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
your repo.

Author: lpcox