Skip test suite on main branch pushes (#4119)

### Description

Every push to `main` runs the full test suite, but we already run them
on merge queue branches in Android. So it's not really needed and wastes
CI credits. This PR disables most jobs from running on `main` except a
few:
- `test_defaults_release` to keep Codecov coverage (Maybe we want to
disable this?)
- Emerge/Paparazzi baseline-upload jobs (So we get diffs in the paywalls
and size checks from main
- prepare-tests (mostly because the other jobs depend on this one)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Direct pushes to main no longer run lint, most tests, or integration
jobs, so regressions could slip through if code reaches main outside the
merge queue.
> 
> **Overview**
> Reduces CircleCI spend on **`main`** by skipping most of the
**`build-test-deploy`** workflow while PR/merge-queue branches still run
the full suite.
> 
> The **`only-main-branch`** anchor is replaced with
**`non-main-branches`** (`branches: ignore: main`), and that filter is
merged into unit tests, lint/detekt/metalava, sample app assembles,
backend/integration/Maestro/UI jobs, and related checks. **`main`**
still runs **`prepare-tests`**, **`test_defaults_release`**
(Kover/Codecov), and the Paparazzi/Emerge baseline jobs
(**`record-and-upload-paparazzi-revenuecatui-snapshots`**,
**`emerge_purchases_ui_snapshot_tests`**,
**`emerge_size_analysis_tests`**) so coverage uploads and snapshot/size
baselines stay current.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
9e3099b74b92270bdf74e44cd1b31a6b9fb3fbb6. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>