fix: build progress releases from current source

[kim-em]

Summary

• remove committed build and egg-info artifacts that could override newer source during packaging
• ignore generated packaging output
• add a regression test rejecting tracked package artifacts
• correct the due-command help text to the eight-hour default

Root cause

The release at 620030c contained current source with an eight-hour cadence, but also a tracked build/lib/progress/plan.py with the old 24-hour cadence. Setuptools could package that stale build tree, so a SHA-pinned uvx invocation still ran old code.

Verification

• full ./tests/run suite passes
• a clean wheel was built and inspected; it contains IDLE_HOURS = 8.0 and the current theorem-first prompt