Add weekly cron workflow for automatic dependency updates

[Copilot]

Implements automatic weekly dependency updates via GitHub Actions cron that runs lake update, rebuilds the catalogue, and creates/updates a PR when changes are detected.

Workflow behavior:

• Runs Mondays at 00:00 UTC (cron: 0 0 * * 1)
• Manual trigger available via workflow_dispatch
• Updates dependencies → builds catalogue → commits only if changes exist
• Reuses single branch auto-update-dependencies for all updates
• Updates existing open PR instead of creating duplicates
• Auto-applies auto update label

Key implementation details:

• Uses --force-with-lease for safe branch updates
• Checks remote branch existence via git ls-remote
• Leverages gh pr list/edit/create for PR management
• Permissions: contents: write, pull-requests: write

The workflow ensures at most one open auto-update PR exists at any time.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Cron Update</issue_title>
<issue_description>GitHub Actionsのcronを使って1週間に1回は最低 lake update を実行して依存関係を更新する．
依存関係に変更があり，lake exe catalogue の実行に成功した場合はそれをコミットしてPRを立てる．
ただしPRが既に経っていてclosedされていない場合はそのPRのタイトルやコミットなどを更新するように．</issue_description>

<agent_instructions>PRには auto update のタグを付けてほしい．</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Cron Update #2

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