Add coding conventions guidance for contributors

Part of #6393.

Problem

CONTRIBUTING.md does not link contributors to any authoritative coding conventions. doc/Developing.md covers building, debugging, and running tests, but not how we expect code to be written (naming, error handling, resource strings, etc.). New contributors have to infer conventions from surrounding code.

Proposed change

Add concise coding conventions guidance - either a new doc/Coding.md referenced from CONTRIBUTING.md, or a dedicated section - covering at minimum:

• Naming and formatting expectations (and a pointer to .editorconfig).
• Error handling patterns used in the codebase (e.g., hr / AICLI_* result handling).
• The resource-string <comment> requirement already documented in doc/Developing.md (Localization).
• A pointer to .github/copilot-instructions.md for build/architecture/convention context.

Notes for engineers

The team should define/ratify the canonical conventions to document here. This issue is about creating the home for them and linking it from CONTRIBUTING.md; the specific rules need engineering input.

Acceptance criteria

• A coding conventions doc (or section) exists and is linked from CONTRIBUTING.md.

(Created with GitHub Copilot assistance.)

Read CONTRIBUTING.md and doc/Developing.md first, then inspect .editorconfig and .github/copilot-instructions.md for the conventions and links already available. Confirm the canonical guidance with engineering input, document the requested naming, formatting, error-handling, and resource-string points, and link the resulting doc or section from CONTRIBUTING.md.