ci: reduce workflows to fork-relevant CI, fix upstream checkouts (Issue #2)

Removed ~37 workflow files that are unmodified upstream tooling this fork
doesn't use (Netlify, SonarCloud, Localazy, release-drafter/backport,
GitHub issue-triage bots, npm-publish, Cloudflare Pages deploy, Docker
push to ghcr.io/element-hq). Enabling Actions for this repo would have
activated all of them simultaneously against a single-capacity shared
runner - most would just fail loudly and starve the one job slot shared
with other repos.

Kept build-and-test.yaml (the actual comprehensive build+test+desktop
workflow) and its build_desktop_*.yaml sub-workflows. Fixed: push trigger
now targets `main` (was `staging`/`master`, neither of which exist here -
push events never fired at all before), 4 checkout steps now check out
this fork instead of element-hq/element-web, and removed the macOS
desktop build job since the registered runner (builder-1) has no macOS
label and would queue forever instead of failing cleanly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/build_desktop_prepare.yaml

@@ -56,7 +56,7 @@ jobs:
             - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
               with:
                   persist-credentials: false
-                  repository: element-hq/element-web
+                  repository: ${{ github.repository }}
 
             - uses: pnpm/action-setup@fc06bc1257f339d1d5d8b3a19a8cae5388b55320 # v5
             - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6