Add GitHub issue templates and triage labels

[Fan1234-1]

Goal: make external feedback arrive in a triageable shape.

Acceptance criteria:

• Add External review, Overclaim report, and Reproducibility report issue templates.
• Each template includes a privacy reminder: no API keys, private chats, personal data, business secrets, or production logs.
• Add status labels: needs-repro, accepted, rejected, needs-more-data, wontfix-explained.
• Do not create a Project Board.