Widen: migrate qeps — including the three status labels QEP-2 retires

[mmcky]

qeps is still on its pre-QEP-2 ad-hoc label set, which includes three status labels the standard retires in favour of native GitHub features — Draft pull requests, ready-for-review, review requests, and issue dependencies.

Retired status labels are the case QuantEcon/cli#37 exists for: their remaining tags are typically all on closed issues and merged pull requests, so deleting them strips history retroactively while nothing on the live tracker changes. That is a decision to take knowingly — delete and accept the loss, or keep the label as deprecated — and it has to be recorded rather than defaulted.

Acceptance

• qeps carries the core 19 and nothing else, or carries a recorded deprecation for any retired label deliberately kept
• Each retired status label has a knowing decision recorded here, with the usage counts on open and closed items that informed it
• The migration report is filed alongside the others