Add ChatGPT App plugin compatibility

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What changed

• Add the standard ChatGPT Plugin manifest and bundled Skill while keeping the root SKILL.md for existing OpenClaw and Claude Code installs.
• Choose scheduling and delivery based on the capabilities available in the current host.
• Report feed failures as partial or error instead of treating them as “no updates.”
• Show a visible data-status line and keep timing claims scoped to each source.
• Remove the delivery script's runtime dependency on node_modules.

Why

Work and Codex use the same ChatGPT Plugin package. The existing feeds and Node scripts already provide the data path, so I kept this as a skills-only Plugin rather than adding an MCP server.

Local installation exposed two issues that affected real Plugin use: installed packages do not include node_modules, and failed or malformed feeds could be mistaken for an empty update. Host testing also showed that an otherwise successful digest could omit its overall status or apply the X lookback window to older blog content.

Compared with #41, this uses the current ChatGPT Plugin layout and does not add proxy behavior without a reproduced host failure.

Tested

• cd scripts && npm test — 11/11 passing
• Plugin and Skill validators — passed
• Installed from a local Marketplace and tested without node_modules
• Live feed fetch — status: ok for X, podcasts, and blogs
• Work fresh-task activation from a natural-language request — passed
• Work native Scheduled Task creation and immediate run — passed
• Fresh Work output after the final Skill update — explicit 数据状态: ok and source-specific time ranges passed

OpenAI documentation: Using plugins and Building plugins.

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