#11315: feat(ci): activate staged pipeline — enable push triggers + ci.yml gates

## Summary **Part 2 of 2** — activates the staged pipeline from #11313. > **Do not merge until:** > 1. PR #11313 is merged > 2. `develop`, `alpha`, and `beta` branches are created > 3. The team is ready for the new workflow ### What this PR does 1. **Modifies `ci.yml`:** - Removes the `push: [main]` trigger (now handled by `release-orchestrator.yml`) - Adds `workflow_call` input so `testing-strategy.yml` can pass `test_stage` - Gates Windows tests to `beta+` staging only (saves compute on regular PRs) - Gates macOS tests to `stable` staging only - Updates concurrency group key 2. **Enables push triggers** on: - `feature-pr.yml` — fires on `dev/**`, `feature/**`, `fix/**` pushes - `hotfix-pr.yml` — fires on `hotfix/**` pushes - `promote-branch.yml` — fires on pushes to `develop`/`alpha`/`beta` - `release-orchestrator.yml` — fires on pushes to `main` 3. **Updates `CONTRIBUTING.md`** — removes "not yet active" caveat from branch strategy ### Impact After merging: - Regular PRs will no longer run Windows/macOS tests (only run at beta/stable promotion) - Pushes to `dev/*` branches will auto-create PRs targeting `develop` - Merging to `main` will trigger the release orchestrator - The full `develop → alpha → beta → main` promotion pipeline goes live ### Testing Workflow changes only — validated by CI on this PR. The `ci.yml` changes can be verified by checking that Linux tests still run on PRs (they do — no gates on Linux jobs).