ci: skip tests if unaffected TASK-1316 (#5295)

### 💭 Notes

Trigger PR workflows based on files affected.

The lists of path matchers corresponding to pytest, npm-test, darker are 
in .github/filters.yml.

For a given CI run, the "Detect changed files" step shows a list of files
changed and which filters were matched by them. You can inspect this on
GitHub:

-> Click "Details" on a PR step 
-> Click the "changes" job
-> Expand the "Detect Changed Files" step
-> Expand individual logs to see:
  - list of files changed
  - list of filters triggered

Ideally, these filters always match whenever a PR has any chance of affecting
a job's pass/fail result.

### 👀 Preview steps

1. 🟢 See the "Checks" section of this PR, or screenshots in the description
2. 🟢 Notice the addition of the "ci / changes" job, which runs first and may
  cause pytest, npm-test, or darker to be skipped if no relevant
  files are changed

.github/filters.yml

@@ -0,0 +1,23 @@
+# Path filters. Used by workflows/ci.yml to decide which jobs to run.
+
+# If any file changed matches any of the filters in a list, the output for that
+# list is set to the string 'true'.
+
+darker:
+  - '{kpi,kobo,hub}/**/*.py'       # .py
+  - 'pyproject.toml'               # rules
+  - '.github/workflows/darker.yml' # ci
+
+pytest:
+  - '{kpi,kobo,hub}/**/*.!(md)'    # backend
+  - 'dependencies/**/*.!(md)'      # pip
+  - 'pyproject.toml'               # (can affect build/tests)
+  - '.github/workflows/pytest.yml' # ci
+
+npm-test:
+  - '{jsapp,test,webpack,static,scripts}/**/*.!(md|py|sh|bash)'   # frontend
+  - '{package*.json,patches/*.patch,scripts/copy_fonts.py}'       # npm + postinstall
+  - '{tsconfig.json,.swcrc,.babelrc*,.browserslistrc}'            # compilers
+  - '{.editorconfig,.prettier*,.stylelint*,.eslint*,coffeelint*}' # linters
+  - '.gitignore'                                                  # (can affect tools)
+  - '.github/workflows/npm-test.yml'                              # ci

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: ci
+
+on: pull_request
+
+jobs:
+  changes:
+    runs-on:     ubuntu-latest
+    permissions: { pull-requests: read }
+    steps:
+      - uses: actions/checkout@v4
+      - id:   filter
+        uses: dorny/paths-filter@v3
+        name: Detect changed files
+        with: { filters: .github/filters.yml }
+    outputs:
+      darker:   ${{ steps.filter.outputs.darker   }}
+      pytest:   ${{ steps.filter.outputs.pytest   }}
+      npm-test: ${{ steps.filter.outputs.npm-test }}
+
+  darker:
+    needs: changes
+    uses: ./.github/workflows/darker.yml
+    if: needs.changes.outputs.darker == 'true'
+
+  pytest:
+    needs: changes
+    uses: ./.github/workflows/pytest.yml
+    if: needs.changes.outputs.pytest == 'true'
+
+  npm-test:
+    needs: changes
+    uses: ./.github/workflows/npm-test.yml
+    if: needs.changes.outputs.npm-test == 'true'

.github/workflows/darker.yml

@@ -1,6 +1,6 @@
 name: Python linter (Darker)
 
-on: [pull_request]
+on: workflow_call
 jobs:
   darker:
     runs-on: ubuntu-latest

.github/workflows/npm-test.yml

@@ -3,7 +3,7 @@ name: npm-test
 on:
   push:
     branches: [ main ]
-  pull_request:
+  workflow_call:
 
 jobs:
   build:

.github/workflows/pytest.yml

@@ -5,7 +5,7 @@ name: pytest
 on:
   push:
     branches: [ main ]
-  pull_request:
+  workflow_call:
 
 jobs:
   build: