MAINT: Remove dead packaging configuration (MANIFEST.in, stale flit excludes, nose relics)

Problem

Configuration from two build-system migrations ago is still on disk, and it is wrong where it isn't ignored:

• MANIFEST.in — flit (our build backend) never reads it, and it references LICENSE.txt where the actual file is LICENSE.
• [tool.flit.sdist] excludes list Makefile and readthedocs.yml, neither of which exists (the real file .readthedocs.yaml is already covered by the .* pattern).
• .coveragerc still omits */site-packages/nose/* and ordereddict.py; .gitignore still lists *.noseids — nose was retired years ago.

Zero runtime impact; nonzero cost to every contributor reverse-engineering how the package actually builds.

Proposed change

Delete MANIFEST.in; prune the two phantom flit excludes; drop nose-era lines from .coveragerc and .gitignore. Then verify packaging is unchanged: build sdist+wheel before and after, diff the file lists (tar tzf / unzip -l).

Acceptance criteria

• Artifact file lists identical pre/post — pure dead weight removed
• grep -ri nose .coveragerc .gitignore empty; no MANIFEST.in

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Start with MANIFEST.in, .coveragerc, .gitignore, and the [tool.flit.sdist] excludes. Build the sdist and wheel before and after the cleanup, then compare file lists with tar tzf and unzip -l; confirm no MANIFEST.in remains and grep -ri nose .coveragerc .gitignore is empty.