Split root Pipfile per test suite for true CI path-filter isolation

[diemol]

Summary

• Follow-up to the path filters added in aecc573: those still listed the shared root Pipfile/Pipfile.lock (and best_practice/conftest.py) in every workflow's paths:, so any dependency bump or shared-fixture edit still triggered every workflow - not real per-directory isolation.
• Gives each test suite its own Pipfile/Pipfile.lock (direct dependencies only, derived from each suite's actual import statements, not copied wholesale from the old root Pipfile) and, where fixtures were shared, its own conftest.py:
  • best_practice/desktop_web/, best_practice/mobile_native/ (android+ios), best_practice/mobile_web/ (vdc+rdc) - each gets a split-off conftest.py with only its own fixtures
  • examples/selenium/, examples/playwright/ - already had their own conftest.py, just get a Pipfile now
  • examples/robotframework/desktop_web/, examples/robotframework/native_mobile/ - no CI coverage for the latter either way, but it gets a minimal Pipfile so its existing pipenv run robot-android-us/robot-ios-us scripts keep working locally
• Every workflow now sets working-directory: to its own suite's directory and its paths: filter is scoped to just that directory + the workflow file itself - no more cross-directory triggering.
• python.yml is removed: its best-practice-desktop-us leg was already fully redundant with desktop_web.yml (both ran the identical script against the same directory), and its demo leg moves to a new selenium_demo.yml.
• Root Pipfile/Pipfile.lock are deleted entirely - no dual-maintenance duplication between a root file and per-suite ones. requirements.txt/.gitpod.yml are untouched (confirmed to be a separate, independent mechanism used only by the Gitpod cloud environment, not by any GitHub Actions workflow).
• sauceclient and sa11y are dropped - confirmed unused by any .py file, workflow, or script anywhere in the repo, so they simply don't appear in any of the new per-suite dependency lists.
• Updated root README.md and best_practice/README.md for the new per-directory cd && pipenv install && pipenv run <script> workflow; also fixed a pre-existing best_practice/README.md bug (missing run in pipenv <script>, and script names that didn't match the real Pipfile, e.g. best-practice-mobile-web-us vs. the actual best-practice-mobile-web-vdc-us).

Test plan

• All 5 workflow YAML files parse cleanly.
• Every new suite's Pipfile.lock was generated via real pipenv lock (not hand-written), each with PIPENV_PIPFILE pinned explicitly so it resolved into the correct directory rather than the (now-deleted) root Pipfile.
• pytest --collect-only passes cleanly for every pytest-based suite (best_practice/desktop_web, best_practice/mobile_native/{android,ios}, best_practice/mobile_web/{vdc,rdc}, examples/selenium, examples/playwright) using each suite's own new conftest.py.
• Verified Robot Framework config/test paths (chrome_config.txt, Tests/, android/android_config.txt, etc.) still resolve correctly relative to each workflow's new working-directory.
• Confirmed no leftover references to the deleted root Pipfile anywhere in workflows or docs.
• Full Sauce-hosted runs weren't exercised for every suite (would require credentials/real device access for all of them) - collection-level verification only.

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