chore(ci): drop macOS and Windows from CI matrix (#60)HEADmain

## Summary

Edify is a pure-Python wheel (`py3-none-any.whl`) with no compiled
extensions, no platform-specific imports, and no filesystem/subprocess
code paths. The multi-OS CI matrix was buying ~zero real signal and
produced false negatives (a transient DNS error on `py312 (macos)`
blocked the v0.3.0 release rerun).

Drop macOS and Windows from the matrix. Linux-only from here on.

## Matrix change

- **Before:** 27 matrix jobs (7 CPython × 3 OSes + 2 PyPy × 3 OSes),
plus `check` and `docs`.
- **After:** 9 matrix jobs (7 CPython × ubuntu + 2 PyPy × ubuntu), plus
`check` and `docs`.
- Diff: 1 file, +1 / -100. Eighteen `windows`/`macos` matrix entries
deleted; no other changes.

`check` and `docs` were already Linux-only and are unaffected. `Analyze
(python)` (CodeQL) and `build` (workflow-level status) are unrelated to
the matrix and remain.

## Branch protection

Already updated in lockstep — `required_status_checks.contexts` on
`main` went from 31 → 13:

```
check, docs,
py38 (ubuntu), py39 (ubuntu), py310 (ubuntu), py311 (ubuntu),
py312 (ubuntu), py313 (ubuntu), py314 (ubuntu),
pypy310 (ubuntu), pypy311 (ubuntu),
Analyze (python),
build
```

Without that pre-update, the removed contexts would block this PR
forever (the protection rule waits for jobs that no longer exist).

## Drafted for your review

Opening as draft. Mark ready when you're satisfied with the diff and
I'll queue auto-merge.

Closes #59

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