docs: document gfx::pipeline + add Markdown style rule to AGENTS.md + README styling

- Added comprehensive "Pipeline Abstraction" section to README.md
  (for dummies, with step-by-step examples, render-to-texture, and
  post-processing).
- Added "Documentation (Markdown)" section to AGENTS.md with the
  max-80-cols rule (exceptions for tables/code blocks).
- Applied consistent Markdown styling to AGENTS.md and README.md
  (wrapped text at 80 cols, bold, code spans, cleaned duplicates).
- Moved scenes libraries to  (cleaned main
  CMakeLists.txt significantly).

No behavior change. All follows AGENTS.md conventions.

AGENTS.md

@@ -12,11 +12,11 @@
 
 ### Commands
 
-  ```sh
-  cmake -S . -B build -GNinja
-  ninja -C build
-  ./build/cuber
-  ```
+```sh
+cmake -S . -B build -GNinja
+ninja -C build
+./build/cuber
+```
 
 ### Dependencies
 
@@ -34,7 +34,8 @@ To add a new dependency:
 
 The project is split into static libraries:
 
-- **`cbt_opengl`** — OpenGL abstraction and window management (window, context, buffer, texture, vao, shader, descriptor)
+- **`cbt_opengl`** — OpenGL abstraction and window management (window,
+  context, buffer, texture, vao, shader, descriptor)
 - **`cbt_scene`** — Base scene class
 - **`scenes_cube`** — Spinning cube scene implementation
 - **`scenes_sphere`** — Cube-to-sphere mapped mesh with diffuse lighting
@@ -52,10 +53,13 @@ to the custom scripts instead of system-installed packages.
 - **No semicolons after closing braces** for namespaces/classes
 - `auto` for obvious types (e.g. `auto main(...) -> int`)
 - **East const** (e.g. `char const*` not `const char*`)
-- **Trailing return type** for all function definitions, including operators (e.g. `auto operator=(T&&) noexcept -> T&`)
-- **Public members first** in class declarations, private members at the bottom
+- **Trailing return type** for all function definitions, including
+  operators (e.g. `auto operator=(T&&) noexcept -> T&`)
+- **Public members first** in class declarations, private members at the
+  bottom
 - `<>` includes only for system headers (std, OS, etc.)
-- `""` includes for third-party dependencies (e.g. `fmt`, `nlohmann/json`)
+- `""` includes for third-party dependencies (e.g. `fmt`,
+  `nlohmann/json`)
 - **Naming:** `snake_case` for variables, functions, and classes
 - **Naming:** `SCREAMING_SNAKE_CASE` only for macros and constants
 - Include order:
@@ -82,21 +86,34 @@ to the custom scripts instead of system-installed packages.
 - Follow the 50/72 rule:
   - Subject line: max 50 characters
   - Body lines: wrapped at 72 characters
-- Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`, etc.)
+- Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`,
+  etc.)
 - Separate subject from body with a blank line
 
 Example:
 
-  ```
-  feat: add stopwatch timer
+```
+feat: add stopwatch timer
 
-  Replace Hello World with a live stopwatch that prints elapsed time
-  in HH:MM:SS.mmm format, updating every 10ms with color output.
-  ```
+Replace Hello World with a live stopwatch that prints elapsed time
+in HH:MM:SS.mmm format, updating every 10ms with color output.
+```
+
+## Documentation (Markdown)
+
+- Wrap normal text and lists at **max 80 columns** (for readability in
+  terminals and editors).
+- **Exceptions**: Tables and code blocks (```` ``` ````) can exceed 80
+  columns when formatting requires it (e.g. trees, alignment).
+- Use standard Markdown: `**bold**`, `` `inline code` ``, `##` headings,
+  `-` or numbered lists, fenced code blocks with language hints
+  (```` ```cpp ````, ```` ```sh ````).
+- Keep examples concise, up-to-date, and self-documenting.
+- This file (`AGENTS.md`) follows its own rules.
 
 ## Source Layout
 
-```
+```text
 cuber/
   CMakeLists.txt      # Build configuration
   cuber.cpp           # Entry point