ci: restore push on all branches and avoid duplicate CI runs (#4665)

Reverts the push trigger from `master`-only back to `"**"` (all branches)
so contributors can see CI status on their fork branches before opening
a PR (see #3840).

To avoid duplicate CI runs when a maintainer pushes to a branch that has
an open PR (both push and pull_request triggers fire), each job now has
an `if` condition that skips pull_request runs from the same repository.
This is the same approach used by Vue.js (vuejs/core):

- Push events: always run
- Pull request events: only run if the PR is from a fork
- Schedule/workflow_dispatch: always run

This avoids the "50% broken CI" visual noise caused by concurrency
groups cancelling push runs, since GitHub shows cancelled jobs as
failures.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>