fix: update plugin source paths in marketplace.json generation

[agreaves-ms]

fix(marketplace): normalize plugin source paths in marketplace.json

Related: #772

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Updated the marketplace generation script to emit bare plugin directory names instead of ./plugins/-prefixed relative paths in the source field. This corrected the source path format across all 46 plugin entries in marketplace.json.

The generation logic in eng/generate-marketplace.mjs previously constructed source paths using a template string (`./plugins/${dirName}`), which produced paths like ./plugins/awesome-copilot. The fix replaced this with the raw dirName value, yielding cleaner paths like awesome-copilot. The regenerated marketplace.json reflects this change uniformly across every plugin entry.

Trailing whitespace on blank lines in the generator script was also cleaned up as part of this change.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• Update to existing instruction, prompt, agent, plugin, or skill.
• Other (please specify):

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Additional Notes

• The marketplace.json output was verified as stable by running npm start and confirming no drift in tracked files.
• This is a focused build tooling fix; no new resources were added or removed.

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