Attach built .jpl artifact to GitHub release (#103)

[Aaravanand00]

Summary

This PR adds a release workflow that automatically builds the plugin and attaches the generated .jpl file to the corresponding GitHub release.

The workflow triggers only when a release is published to avoid duplicate runs from tag pushes.

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What It Does

• Runs on release: published
• Uses npm ci for deterministic dependency installation
• Builds the plugin using npm run dist
• Attaches publish/joplin.plugin.templates.jpl to the release
• Fails safely if the artifact is not generated
• Includes concurrency control to prevent overlapping runs

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Safety Measures

• No push tag trigger (avoids duplicate builds)
• Deterministic install via npm ci
• fail_on_unmatched_files: true ensures release fails if build artifact is missing
• Minimal, self-contained workflow
• Does not modify production code

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Local Validation

• Clean install succeeded
• .jpl file generated successfully
• Artifact path verified
• Workflow YAML validated

Please let me know if you'd prefer running tests before packaging as part of the release workflow.