📝 docs(config): update CacheManager documentation with usage guidelines and antipatterns

**Single Source of Truth:** All file paths MUST use `CacheManager` from `src/tdoc_crawler/config/__init__.py`.

The CacheManager must be registered once at the start of the program, and then resolved wherever needed.

### Usage

```python

manager = resolve_cache_manager()

# Or create new (auto-registers)

manager = CacheManager().register()

manager = CacheManager(cache_dir).register()

# Access paths (NEVER hardcode these)

manager.root              # ~/.3gpp-crawler/

manager.ai_embed_dir("qwen3-embedding-0.6b")

```

In the current framework, the `CacheManager` is instantiated only the the CLI wrapper.

If used as a library, the user must create and register their own instance *as soon as possible* at the start of their program. Any method/class relies on a properly registered `CacheManager` being available - fallback/try-except-boilerplate must not be used!

```python

# ❌ WRONG - boilerplate/too much safety - just let it fail if not registered, it's a dev error that must be fixed

try:

    manager = resolve_cache_manager()

except CacheManagerNotRegisteredError:

    try:

        manager = CacheManager(default_cache_dir).register()

    except Exception as e:

        raise RuntimeError("Failed to create and register CacheManager. Please ensure it's registered at the start of your program.") from e

    raise RuntimeError("CacheManager must be registered before use. Please create and register an instance at the start of your program.")

# ✅ CORRECT - simply resolve it, without an argument. Let it burn if not registered!

manager = resolve_cache_manager()

```

## Antipaterns (what NOT to do)

Errors are often masked by trying to be too clever and/or too careful with error handling or by not following the established patterns. Always prefer simplicity and clarity over complex workarounds. Examples:

```python

# ❌ WRONG - arguments and result types may be None to handle invalid inputs or to indicate failed operation

def get_info(number: str|int|None, message: str|None|Any) -> InfoObject|str|None:

    # This function tries to handle too many cases and returns different types, making it hard to use and error-prone. It also uses None in multiple ways, which can lead to confusion.

    if isinstance(tdoc_id, None):

        raise ValueError("tdoc_id cannot be None")

    if not isinstance(tdoc_id, (str, int)):

        raise TypeError("tdoc_id must be a string or integer")

    # ... rest of the logic

    try:

        # some processing logic that may fail

        ...

        return info_object  # on success

    # "encode" logic into return values, which is an antipattern. It makes it hard for users to know what to expect and how to handle different cases.

    except SomeSpecificError:

        return None

    except AnotherError:

        return "Error: Invalid input"  # on invalid input

# ✅ CORRECT - keep it simple/clear, with consistent return types, minimum amount of checking. Otherwise: let it burn!

def get_info(number: str|int, message: str) -> InfoObject:

    if not isinstance(tdoc_id, (str, int)):

        raise TypeError("tdoc_id must be a string or integer")

    # some processing logic that may fail

    ...

    return info_object  # on success

```

let it burn if not registered

## Scoped AGENTS.md (MUST read when working in these directories)

| Directory | Purpose |