Feature request: support explicit document includes in CodeRabbit path instructions

## Summary

Support an explicit file-reference/include syntax inside `reviews.path_instructions[].instructions` so repository guidance documents can be attached as review context without relying on natural-language phrasing.

## Motivation

Repository configuration currently needs prose such as:

```yaml
instructions: |
Treat `docs/guidance/typescript.md` as required review context.
```

A concise, unambiguous directive such as the following would be easier to maintain and validate:

```yaml
instructions: |
@docs/guidance/typescript.md
```

At present, this notation is interpreted as plain text rather than an instruction to load the referenced document.

## Affected area

CodeRabbit configuration parsing and review-context routing for `reviews.path_instructions[].instructions`.

## Acceptance criteria

- A documented syntax exists for referencing repository files from `path_instructions[].instructions`.
- Referenced files are loaded as review context for changed paths matching the rule.
- Invalid or missing references result in clear configuration validation or review diagnostics.
- The syntax works alongside ordinary natural-language instructions.
- Examples are added to the CodeRabbit configuration documentation/schema as appropriate.

Requested by: @DrJKL

Backlinks:
- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13577
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13577#discussion_r3633975220