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

AGENT.md

@@ -11,16 +11,17 @@ Decade-scale maintainability. Only essential tech. Readable, simple, future-proo
 1. **Building on top** (`custom/`): Create sites using the framework. Never modify `app/`. In this mode, `app/` is typically symlinked or submoduled from the framework repo into a separate site repo.
 2. **Framework development** (`app/`): Evolve the core. Preserve all stable contracts (see architecture doc). `custom/` may be symlinked in from a site repo for testing.
 
-## Core Constraints
+## Framework Development Workflow
+
+When modifying or adding code in `app/`: write tests and run them before marking work done. Read `docs/05-testing/01-testing.md` for the required workflow and test format.
+
+## Standards
 
 - **Stack:** HTML5, PHP 8.4+, CSS. Nothing else.
-- **Frontend:** Classless semantic HTML, modern CSS (nesting, `oklch()`, grid, `clamp()`, logical props)
 - **Security:** Path traversal protection, document root restriction, strict MIME types, escape all UGC
-
-## Code Style
-
-- **PHP:** Arrow functions, null coalescing, match expressions. Type hints where practical. Single-purpose functions. Comments only for major sections.
-- **CSS:** Variables, native nesting, grid. `clamp()` over `@media`. Relative units.
+- **PHP:** Arrow functions, null coalescing, match expressions. Type hints where practical. Single-purpose functions, avoid side effects. Comments only for major sections.
+- **HTML:** Classless, semantic markup.
+- **CSS:** Variables, native nesting, relative units. Modern CSS (`oklch()`, `light-dark()`, `clamp()`, logical props). Global styles first, page-scoped when needed.
 - **Templates:** `<?= htmlspecialchars($var) ?>` for UGC. `<?= $content ?>` for pre-rendered HTML.
 
 ## Knowledge Base
@@ -37,5 +38,6 @@ Read these docs on-demand when working on related areas. Do not load all at once
 | Templates | `docs/04-development/06-templates.md` | Template hierarchy, resolution, variables, list item schema, partials |
 | Rendering | `docs/04-development/07-rendering.md` | Rendering pipeline, Markdown caching, static file serving, Parsedown |
 | Dev Environment | `docs/04-development/08-dev-environment.md` | Container setup, Apache config, performance profiling, test data generation |
+| Testing | `docs/05-testing/01-testing.md` | **Required before modifying `app/`** — workflow, test format, what's testable |
 
 Human-facing docs (tutorials, reference) are in `docs/01-getting-started/`, `docs/02-tutorial/`, `docs/03-reference/`.