fix(ci): don't run desktop tests for purely mobile client changes (#7709)

Mobile-only PRs currently trigger desktop CI because the desktop test
filter's glob inadvertently matches every path outside Tauri.

Fix that filter so that pure mobile changes skip desktop builds, tests,
and their relay artifact producer.

### Verification

- 12 regression tests exercise the exact pinned paths-filter action on
mobile, desktop, Tauri, backend, mixed, workflow, and documentation
changes.
- Restoring the bad glob makes the mobile-only regression test fail.
- The filter tests, existing required-context isolation contract,
workflow syntax validation, and Biome pass locally.

---------

Signed-off-by: Tom Brow <tomb@block.xyz>

.github/workflows/ci.yml

@@ -63,7 +63,6 @@ jobs:
               - 'scripts/model-capabilities.json'
               - 'scripts/normative-corpus.json'
               - 'desktop/**'
-              - '!desktop/src-tauri/**'
               - 'pnpm-lock.yaml'
             desktop-rust:
               - 'desktop/src-tauri/**'
@@ -114,6 +113,8 @@ jobs:
           scripts/test-rust-cache-contract-regressions.sh
       - name: CI required-context isolation contract
         run: scripts/test-ci-required-context-isolation.sh
+      - name: CI path selection regression tests
+        run: node --test scripts/ci-selection.test.mjs
       - name: File size policy
         run: just file-size-check