feat: Add /unassign command to allow contributors to unassign themselves

[darshit2308]

Description:
Add support for the /unassign command to allow contributors to drop issues they no longer have bandwidth for, reducing manual maintainer overhead.

• Add removeAssignees helper to helpers/api.js
• Implement /unassign core logic (commands/unassign.js) validating issue state, authorization, and label reversion
• Create dedicated comment builders (commands/unassign-comments.js)
• Update /assign welcome comment to inform users about the new /unassign feature
• Hook /unassign route into the comment event dispatcher (bot-on-comment.js)

Related issue(s):
Fixes #1239

Notes for reviewer:

• Tested locally using a new test-unassign-bot.js suite that mirrors the existing custom mock framework.
• The new tests provide full coverage for happy paths, authorization gates (including case-insensitive username matching), edge cases, and API failures.
• API label failures are logged server-side rather than silently failing to ensure system observability.

Checklist

• Documented (Code comments)
• Tested (unit, integration, etc.)

[github-actions]

Hey @darshit2308 👋 thanks for the PR!
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PR Checks

✅ DCO Sign-off -- All commits have valid sign-offs. Nice work!

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✅ Issue Link -- Linked to #1239 (assigned to you).

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🎉 All checks passed! Your PR is ready for review. Great job!

[rwalworth]

LGTM - clean, well-structured implementation that follows the existing patterns nicely. All the requirements from #1239 are covered (authorization gates, edge cases, removeAssignees helper, file structure, welcome comment update) and the test suite is thorough.

A few overall formatting items to clean up:

• Several files are missing the trailing newline at end of file (bot-on-comment.js, unassign.js, unassign-comments.js, test-assign-bot.js, test-unassign-bot.js). Note that bot-on-comment.js and test-assign-bot.js previously had trailing newlines - please restore those, and add them to the new files as well.
• There's trailing whitespace on a few lines (bot-on-comment.js lines 12 and 36, unassign.js line 89, test-unassign-bot.js line 13). A quick pass should catch these.

I left a couple more notes below - nothing blocking. Running workflows now and will merge once they pass!

Congratulations on your first contribution to the Hiero C++ SDK @darshit2308! 🎉 Thank you for taking the time to contribute!

[PavelSBorisov]

LGTM