GH workflows only trigger for relevant parts of the code base

[douglowe]

The unit test, integration test, and code linting workflows only run on changes to the app and tests directories:

on:
  pull_request:
    branches: [ develop ]
    paths:
      - app/**
      - tests/**

[Copilot]

Pull request overview

This PR narrows when GitHub Actions workflows run on pull_request events by adding paths filters, aiming to trigger CI only when relevant parts of the repository change.

Changes:

• Added on.pull_request.paths filters to unit tests, integration tests, and lint workflows.
• Limited workflow triggers to changes under app/** and tests/**.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/unit_tests.yml	Restricts unit test workflow triggers to app/** and tests/**.
.github/workflows/test_docker.yml	Restricts integration test workflow triggers to app/** and tests/**.
.github/workflows/lint.yml	Restricts lint workflow triggers to app/** and tests/**.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[alexhambley]

This made me look at our CI coverage more broadly, and I'd like us to consider a few things before merging:

1. The workflows can't trigger themselves, so a PR that only touches .github/workflows/ (e.g., changing these filters, or bumping the Python version) would run no CI at all. I'd suggest adding .github/workflows/** to each list.

2. We currently have no CI on merges. These workflows only run on pull_request, so nothing runs when a PR reaches develop/main. This was OK before these changes, but with path filtering added on top, we need a push trigger as a safety net, e.g something like:

on:
  push:
    branches: [ develop, main ]
  pull_request:
    branches: [ develop, main ]
    paths:
      ...

Another thing (maybe for issue #193 rather than here): if we ever make these checks required, fltered workflows never report a status, so docs-only PRs for example would sit on "expected" forever.