Add bd reopen command for explicit issue reopening
Implements bd-235: New reopen command makes reopening issues more explicit
than 'bd update --status open'.
Features:
- Sets status to 'open' and automatically clears closed_at
- Emits EventReopened via UpdateIssue
- Optional --reason flag to add comment
- JSON output support
- Auto-flush after operation
Closes: bd-234, bd-235, bd-236, bd-237, bd-238, bd-239
Related: bd-224 (status/closed_at invariant enforcement)
Amp-Thread-ID: https://ampcode.com/threads/T-b1c82629-3521-440f-9f16-ab6d66fb5091
Co-authored-by: Amp <amp@ampcode.com>
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