feat: add CAJAL scientific paper generator custom mode example

[roomote-v0]

This PR attempts to address Issue #12256.

Summary

Adds a ready-to-use .roomodes file and documentation for the CAJAL scientific paper generator mode, following the agreed-upon approach of starting with a distributable custom mode before considering a built-in mode.

What is included

• docs/community-modes/cajal-paper-generator/.roomodes -- A complete custom mode definition that users can copy into their project root to enable the Paper Generator mode
• docs/community-modes/cajal-paper-generator/README.md -- Documentation covering installation, usage, the tribunal workflow, and configuration options

How it works

The paper mode defines a structured workflow for generating scientific papers:

1. Pass 1 (Draft): Generates a 7-section paper (Abstract, Introduction, Related Work, Methodology, Results, Discussion, Conclusion) with real arXiv citations
2. Pass 2 (Tribunal): Simulates 3 independent reviewers scoring each section (0-10)
3. Pass 3 (Refinement): Rewrites sections scoring below 7, up to 3 iterations

The mode uses file restrictions to only write .md, .tex, .bib, .txt, .csv, and .json files, preventing accidental source code modifications.

No core code changes

This is purely additive documentation -- no changes to extension source code, tests, or configuration. The mode works through the existing custom modes system (.roomodes files) and requires zero new infrastructure.

Testing

• YAML parses correctly and validates against the .roomodes schema
• Existing CustomModesSettings and modes test suites pass (14 + 45 tests)
• All linting and type checks pass

Feedback and guidance are welcome.

Refs: #12256

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