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/config/.files.yaml

@@ -1,16 +1,34 @@
+# CI routing infra. Editing the top-level router (build.yml) or this filter
+# config re-runs every area's jobs, so every job-gating filter below includes
+# *ci. That makes a change to how jobs are dispatched actually exercise those
+# jobs (self-testing), instead of a router edit only matching the project filter.
+ci: &ci
+  - .github/workflows/build.yml
+  - .github/config/.files.yaml
+
 build: &build
+  - *ci
   - build.gradle
   - app/(common|core|proprietary|saas)/build.gradle
   - Taskfile.yml
   - .taskfiles/backend.yml
+  - .github/workflows/check-licence.yml
 
 openapi: &openapi
+  - *ci
   - *build
   - app/(common|core|proprietary|saas)/src/main/java/**
+  - .github/workflows/check-openapi.yml
 
 docker-base: &docker-base
   - docker/base/Dockerfile
 
+# Dockerfiles only (base + embedded + unoserver). Gates the slow multi-arch
+# (arm64) leg of the PR docker test build: arm64 is only rebuilt when a
+# Dockerfile itself changes, not on every code PR.
+dockerfiles: &dockerfiles
+  - docker/**/Dockerfile*
+
 docker: &docker
   - docker/embedded/Dockerfile
   - docker/embedded/Dockerfile.fat
@@ -23,13 +41,11 @@ docker: &docker
   - *docker-base
 
 project: &project
+  - *ci
   - app/(common|core|proprietary|saas)/src/(main|test)/java/**
   - *build
   - "app/(common|core|proprietary|saas)/src/(main|test)/resources/**/!(messages_*.properties|*.md)*"
   - exampleYmlFiles/**
-  - gradle/**
-  - libs/**
-  - "testing/**/!(requirements*.txt|requirements*.in)*"
   - *docker
   - *docker-base
   - gradle.properties
@@ -45,8 +61,11 @@ project: &project
   - .taskfiles/docker.yml
   - scripts/db-migration/**
   - .github/workflows/db-migration-test.yml
+  - .github/workflows/docker-compose-tests.yml
+  - .github/workflows/test-build-docker.yml
 
 frontend: &frontend
+  - *ci
   - frontend/**
   - .github/workflows/testdriver.yml
   - testing/**
@@ -63,10 +82,14 @@ frontend: &frontend
   - Taskfile.yml
   - .taskfiles/frontend.yml
   - .taskfiles/e2e.yml
+  - .github/workflows/frontend-validation.yml
+  - .github/workflows/e2e-stubbed.yml
+  - .github/workflows/e2e-live.yml
 
 # Files that affect the Tauri desktop bundle. Gate the multi-OS Tauri build
 # job on changes to any of these.
 tauri: &tauri
+  - *ci
   - frontend/editor/src-tauri/**
   - frontend/editor/src/desktop/**
   - frontend/editor/tsconfig.desktop.vite.json
@@ -81,6 +104,7 @@ tauri: &tauri
 # the engine validation job on changes to engine sources or to the Java
 # tool surfaces it generates models from.
 engine: &engine
+  - *ci
   - engine/**
   - app/(common|core|proprietary|saas)/src/main/java/**
   - .github/workflows/ai-engine.yml
@@ -93,6 +117,7 @@ engine: &engine
 # tasks that drive generation. Deliberately excludes the broad frontend/docker/
 # testing globs, so a CSS-only PR does not boot the backend to rebuild the spec.
 generated-models: &generated-models
+  - *ci
   - *openapi
   - frontend/editor/scripts/generate-tool-api-types.mts
   - frontend/editor/src/core/types/toolApiTypes.ts
@@ -115,6 +140,7 @@ licenses-backend: &licenses-backend
 # Files that can affect premium / enterprise behaviour. Gate the enterprise
 # Playwright job on changes to any of these on PRs.
 proprietary: &proprietary
+  - *ci
   - app/proprietary/**
   - frontend/editor/src/proprietary/**
   - frontend/editor/src/core/tests/enterprise/**