Add fallback issue template

[bingran-you]

Summary

• add a classic fallback issue template in addition to the existing issue forms
• keep credential/security guidance visible when GitHub falls back to the classic issue flow

Validation

• git diff --check
• Markdown local link scan: missing=0

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Move fallback template into ISSUE_TEMPLATE

This file is being added as .github/ISSUE_TEMPLATE.md, but GitHub's issue-template docs state that issue templates are loaded from .github/ISSUE_TEMPLATE; with the existing form files already in that directory, someone choosing the blank/classic issue path will still get an empty body, so the credential/security warning here won't be shown. Put the fallback Markdown under .github/ISSUE_TEMPLATE/ (or add a chooser config/contact link) so GitHub actually applies it.

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