Previously the OLM catalog index (olm/index-template.yaml) was stored
on a separate long-lived olm-catalog branch and updated by committing
directly to it on every CI run.

This change replaces that pattern with a PR-based approach:
- olm/ is moved into main (and release-* branches) as a regular
  tracked directory
- The olm-publish action no longer performs any git operations; it
  only builds and pushes images, leaving olm/index-template.yaml
  modified in the working tree
- Each release workflow opens (or force-updates) a PR with the
  catalog changes for human review before they land

Channel/trigger/branch mapping:
  development  push to main                     -> PR to main
  candidate    chore(release): on release-*     -> PR to release-X.Y
  stable       GitHub release promoted          -> PR to release-X.Y

New release-branch.yaml handles RC releases (prerelease creation,
OLM candidate publish, catalog PR) cut from release-* branches.

Also updates docs/developer.md and docs/design/release.md to reflect
the new release process.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Jan Fajerski <jfajersk@redhat.com>