Reduce CI cost: disable Depot, gate arm64/Tauri PR builds, self-testing CI routing (#7028)

## What

CI cost/routing cleanup. Four changes, each reversible with no code
deleted.

### 1. Disable Depot repo-wide (reversible)
Depot ran on trusted (non-fork) triggers via the `is_fork` output of
`_runner-pick.yml`, driving both the `depot-*` runner selection and the
Depot docker build actions. It's now disabled everywhere behind a single
kill-switch:

- `_runner-pick.yml` gains a dedicated `use_depot` output, forced
`false` via `DEPOT_ENABLED=false`. `is_fork` stays truthful for trust
gating (e.g. `build-enterprise` skipping on forks).
- All `runs-on:` and `USE_DEPOT:` expressions now key off `use_depot`,
so every job falls back to `ubuntu-latest` + buildx.
- `settings.gradle` Depot remote build cache (`cache.depot.dev`) gated
behind `depotCacheEnabled = false`.

**Switch back on:** set `DEPOT_ENABLED=true` in `_runner-pick.yml` (and
`depotCacheEnabled = true` in `settings.gradle`). Depot then reactivates
on trusted triggers exactly as before.

### 2. arm64 PR docker build only on Dockerfile changes
`test-build-docker.yml` was building `linux/amd64,linux/arm64/v8` on
every PR matching the broad `project` filter. With Depot off, the arm64
leg runs under slow QEMU emulation on every code PR. New `dockerfiles`
path filter (`docker/**/Dockerfile*`) gates the arm64 leg: normal code
PRs build amd64 only; PRs that touch a Dockerfile still build amd64 +
arm64. arm64 is still fully exercised on the base-image publish and on
release.

### 3. Tauri PR build -> Linux only, unsigned, deb-only
The PR path built the full 3-OS matrix (Windows + macOS-universal +
Linux), plus the flaky Linux AppImage pass (#6127). PRs now build Linux
only (fastest + cheapest to compile) via a new `minimal` input on
`tauri-build.yml`: Linux deb only, no rpm, no AppImage. The full signed
multi-OS matrix still runs on release, and nightly still warms the Rust
cache with all-OS defaults (unchanged).

Tradeoff: Windows/macOS desktop build breaks are caught by nightly
(all-OS) rather than the introducing PR.

### 4. CI self-testing routing
Editing `build.yml` only matched the `project` filter, so a change to
how e2e / enterprise / tauri / engine jobs are dispatched didn't
actually run those jobs. Added a `ci` anchor (`build.yml` +
`.github/config/.files.yaml`) that every job-gating area filter now
includes, so editing the router or the filter config runs every job.
Also added the orphaned reusable workflows (`e2e-*`,
`frontend-validation`, `docker-compose-tests`, `test-build-docker`,
`check-openapi`, `check-licence`) to their area filters so editing a
reusable workflow self-tests.

## Validation
- All workflow YAML + `.files.yaml` parse; anchor resolution verified
(every job-gating filter resolves to include the `ci` paths).
- Gradle evaluates `settings.gradle` cleanly; `spotlessGradleCheck`
passes.

.github/workflows/check-openapi.yml

@@ -15,7 +15,7 @@ jobs:
 
   check-generate-openapi-docs:
     needs: pick
-    runs-on: ${{ needs.pick.outputs.is_fork == 'true' && 'ubuntu-latest' || 'depot-ubuntu-24.04-4' }}
+    runs-on: ${{ needs.pick.outputs.use_depot == 'true' && 'depot-ubuntu-24.04-4' || 'ubuntu-latest' }}
     env:
       DEPOT_TOKEN: ${{ secrets.DEPOT_TOKEN }}
     steps: