chore: retire local issue/discussion templates -> inherit org .github canon

Local .github/ISSUE_TEMPLATE / DISCUSSION_TEMPLATE duplicated the generic
templates now provided org-wide by hyperpolymath/.github. Removing the local
folder lets this repo inherit the canonical set. Reversible; no licence changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath