test(ci): add fish config test suite and gate docs build on it

Adds tests/run-tests.fish (syntax lint over every .fish file, plus a
sandboxed interactive load) and tests/functional.fish (10 checks
covering XDG/PATH/CDPATH setup, key bindings, abbreviations, core
functions, exit rewiring, and the opinionated-component registry).

The sandbox copies config-relevant files into a scratch HOME/XDG tree
rather than symlinking the checkout, since this repo also serves as a
live ~/.config/fish and a symlink would let universal-variable writes
leak into the real fish_variables file.

Wires the suite into build-docs.yml as a `test` job that `build-docs`
now depends on, so a broken config can no longer get published to the
docs site. Documents the workflow in the README's new Testing section.

.github/workflows/build-docs.yml

@@ -13,10 +13,33 @@ on:
       - "functions/**"
       - "conf.d/**"
       - "config.fish"
+      - "completions/**"
+      - "integrations/**"
+      - "tests/**"
   workflow_dispatch:
 
 jobs:
+  test:
+    runs-on: racknerd-mini
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          token: ${{ secrets.GITEA_TOKEN }}
+
+      - name: Install fish
+        run: |
+          sudo apt-get update -qq
+          sudo apt-get install -y software-properties-common
+          sudo add-apt-repository -y ppa:fish-shell/release-4
+          sudo apt-get update -qq
+          sudo apt-get install -y fish
+
+      - name: Run fish config tests
+        run: fish tests/run-tests.fish
+
   build-docs:
+    needs: test
     runs-on: racknerd-mini
     steps:
       - name: Checkout