Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

docs/05-testing/01-testing.md

@@ -0,0 +1,73 @@
+# Testing
+
+## Workflow
+
+When modifying or adding functions in `app/`, follow this sequence — **a task is not complete until all steps are done**:
+
+1. **Write or update the code** in the relevant `app/*.php` file.
+2. **Write tests** in `devel/tests/<module>/` matching the changed file (e.g. `app/content.php` → `devel/tests/content/`).
+3. **Run the tests:** `cd devel && ./tests/run.sh`
+4. **Fix any failures** — do not consider the task done while tests are red.
+
+When tests already exist for the changed function, update them to cover the new behaviour.
+
+**Not unit-testable** (skip for these): `router.php`, `static.php`, template files, full HTTP flows.
+
+## Framework Tests (`app/`)
+
+Unit tests live in `devel/tests/` and use the [phpt format](https://php.github.io/php-src/miscellaneous/writing-tests.html) ([lessons learned](https://wiki.php.net/qa/phptlessonslearned)). The runner is a minimal PHP script — no test framework, no Composer. Tests cover `app/` only.
+
+### Running
+
+```bash
+cd devel
+
+./tests/run.sh              # Run all tests
+./tests/run.sh helpers      # Filter by subdir or filename substring
+```
+
+`CONTAINER=my-container ./tests/run.sh` to override the container name. Exit code `1` on any failure — CI-compatible. Container must be running (`podman-compose up -d`).
+
+### File Layout
+
+```
+devel/tests/
+  run.php           # Runner — executed inside the container
+  run.sh            # Host entry point (calls podman exec)
+  helpers/          # Tests for app/helpers.php
+  content/          # Tests for app/content.php
+  config/           # Tests for app/config.php
+  ...               # One subdir per app/ module
+```
+
+### Writing Tests
+
+App files are at `/var/www/app/` inside the container. Require only what the function under test needs.
+
+```
+--TEST--
+extractRawDateFromFolder: extracts YYYY-MM-DD from date-prefixed folder names
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+echo extractRawDateFromFolder('2024-01-15-my-post');
+?>
+--EXPECT--
+2024-01-15
+```
+
+Filesystem tests: use `sys_get_temp_dir() . '/phpt_' . getmypid()` and clean up in `--FILE--`.
+
+### What to Test
+
+| Test | How |
+|---|---|
+| Pure functions | Direct call + echo |
+| Filesystem functions | Temp dir, clean up after |
+| Config merging, metadata parsing | Temp files or inline strings |
+| Full HTTP flow | Browser or `perf.sh` — not unit-tested |
+| Template rendering | Not unit-tested |
+
+## Site Tests (`custom/`)
+
+For sites built on top of the framework, [Pest](https://github.com/pestphp/pest) is a good fit — it supports browser testing via Playwright, useful for end-to-end testing of custom templates, plugins, and content. Pest is not used in this repo.