🤖 ci: use paths-filter for selective test execution

Use dorny/paths-filter to detect file changes and skip tests when not needed.

Changes:
- Add 'changes' job to detect docs-only and browser-only PRs
- Jobs use simple job-level 'if:' to skip when not needed
- Add 'ci-passed' job as single required check (passes if all jobs pass or skip)
- Simplify workflow by removing verbose step names and comments

Skip logic:
- docs-only: skip all tests (unit, integration, e2e, storybook, smoke)
- browser-only: skip integration tests (no backend impact)
- config changes: always run all tests (safety)

The 'ci-passed' job should be configured as the only required check in branch
protection settings. It will pass when jobs are skipped due to path filtering.

_Generated with `mux`_

Author: ammar-agent