docs: fix e2e skip mechanism description, add CI and caching sections

Correct inaccurate claim that addopts excludes e2e tests by marker
filter. The actual mechanism is pytestmark skipif conditions that check
for env vars at import time. Add CI workflow section documenting the
two-step test strategy and uv cache behavior.

Author: pythoninthegrass

docs/testing.md

@@ -3,7 +3,7 @@
 ## Running tests
 
 ```bash
-# Run tests (e2e excluded by default via addopts)
+# Run all tests (e2e tests auto-skip when env vars are absent)
 uv run pytest
 
 # Run a single test file or test
@@ -18,6 +18,7 @@ uv run pytest -m e2e
 
 - `asyncio_mode = "auto"` -- async test functions run without `@pytest.mark.asyncio`
 - `--cov` is on by default via `addopts`
+- `addopts` does **not** filter by marker -- e2e tests are collected but skipped via `skipif`
 
 ## Test categories
 
@@ -44,4 +45,26 @@ E2E tests require the following environment variables:
 - `TAILSCALE_API_KEY`
 - `TAILSCALE_TAILNET`
 
-These are excluded by default via pytest `addopts`. Run with `uv run pytest -m e2e`.
+E2E tests use `pytestmark` with two `skipif` conditions that check for the env vars
+at import time via `python-decouple`. When either var is absent, all tests in the
+file are skipped (shown as `s` in output). This means `uv run pytest` collects e2e
+tests but skips them -- no marker filter is needed.
+
+Run only e2e tests with `uv run pytest -m e2e`.
+
+## CI workflow
+
+The GitHub Actions workflow (`.github/workflows/pytest.yml`) runs on Blacksmith
+runners and has two test steps:
+
+1. **Run unit tests** (`uv run pytest -s`) -- collects all 78 tests; e2e tests
+   auto-skip because the secrets env vars are not injected into this step.
+2. **Run e2e tests** (`uv run pytest -m e2e -s`) -- runs only on `push` (not PRs)
+   with `TAILSCALE_API_KEY` and `TAILSCALE_TAILNET` injected from repo secrets.
+
+### Caching
+
+`astral-sh/setup-uv@v6` caches the uv download cache (`~/.cache/uv`) by default.
+The `.venv` is recreated each run but packages are linked from the cache, so
+`uv sync --all-extras` typically completes in ~1 second. Caching the venv itself
+is not currently necessary given the fast install times.