CI/CD optimization and repository health improvements

**Description:**

This issue tracks workflow optimizations and repository-level improvements. The primary goal is reducing MAC37 queue time and cleaning up committed artifacts.

### `runs-at-night.yml`

- [ ] No functional changes needed — file is fully commented out; either restore it or delete it to avoid dead-code confusion

### `deploy-docs-to-staging.yml`

- [ ] Delete workflow file entirely

### `push.yml`

- [ ] Restrict to run only on auto-update branches (currently fires on every push to every branch, including auto-update branches created by `transfer_to_open_source.yml` in the closed-source repo)
- [ ] [TBD] Move `Roslyn-Analyzer-dotnet` job from `MAC37` to a Windows runner — it only needs dotnet, not macOS
- [ ] [TBD] Move `copyright` job from `MAC37` to a Windows runner — it only runs a PowerShell script
- [ ] Add `timeout-minutes: 60` to all build/test jobs
- [ ] Add `actions/cache` for `Library/` keyed on `Assets/**`, `Packages/**`, `ProjectSettings/**` — currently every run reimports Unity assets from scratch

### Runner Distribution

- [ ] Change all `runs-on: [self-hosted, MAC37]` to `runs-on: [self-hosted, macOS]` for jobs that don't require Mac37 specifically — this allows Mac36 to share the load

### Repository Hygiene

- [ ] Remove committed `.DS_Store` files — listed in `.gitignore` but committed before the rule was added; run `git rm --cached $(git ls-files -i --exclude-standard)` and commit
- [ ] Remove committed `.venv/` directory — a full Python virtual environment is tracked in source control; run `git rm -r --cached .venv/`
- [ ] Verify `Library/` folder is not tracked — it exists in the working tree; confirm it is fully excluded by the `.gitignore`
- [ ] Make sure that there is a single file as source of truth for version
- [ ] Standardize issue templates to match the shared template set