Rewrite the README around the pipeline as it runs today

The README described the repository host as the publisher: bin/repo
release as the main workflow, five-minute systemd timers, a signing key
that lives on the host and nowhere else, aarch64 under emulation. Since
September 18 packages are built by build-pr.yml and published by
publish.yml, and neither workflow was mentioned. ci/README.md still opened
with "Status: spike. Nothing here publishes."

The README now covers, in order: contributing a package, how a package
reaches users, where it is published, the maintainer tasks (republish,
build, retire, check that a version is live, read the publish log), what
to do when a publish fails, who can build and merge, the three scheduled
update workflows, package metadata, releases, and the rules that keep two
publishers from colliding.

Text about the old host pipeline moved to docs/repository-host.md, marked
abandoned. The rest moved to docs/ largely unchanged:
- docs/releases.md: the release train and versioning rules
- docs/rebuild-triggers.md: rebuild_on and bin/sync-rebuilds
- docs/builds.md: dependency order, isolation, builder images
- docs/upstream-sources.md: gains the provider and hook reference

ci/README.md is rewritten as the operating guide for the x86_64 builder
pool. Statements the move would have carried over wrong are corrected,
among them the signing key's location, AUR sync, and which architecture
list CI reads. bin/sync-upstream's help now points at the docs page that
holds the provider schemas.

bin/sync-upstream

@@ -23,7 +23,7 @@ Update packages that track an upstream vendor release feed instead of the AUR.
 Packages opt in declaratively through "upstream" in .omarchy/package.json.
 Providers cover GitHub Releases with checksum manifests or API asset digests,
 semver-shaped git tags, npm dist-tags, and plain Debian Packages indexes. See
-README.md for the schemas. Anything outside those conventions may provide
+docs/upstream-sources.md for the schemas. Anything outside those conventions may provide
 pkgbuilds/<package>/.omarchy/upstream.sh, a hook that reports JSON on stdout:
 
   {