Check for existing issues before submitting feedback

When submitting a bug or feature request via `/feedback`, check if a similar issue already exists. If a potential duplicate is found, return the link and ask the user to confirm: either "this is a duplicate — append a comment saying I've experienced the same issue" or "not a duplicate — submit as a new issue." This avoids creating duplicate issues and helps surface popular requests.

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### Metadata

| Field | Value |
|-------|-------|
| CLI Version | v0.4.1 |
| Platform | darwin |
| Architecture | arm64 |
| OS Release | 25.3.0 |
| Category | improvement |
| Working Directory | AI-Tools |
| Session ID | ses_3030e1e46ffey2pcMRovdii3tK |