docs: rewrite contributing guide (AGENTS.md) to clarify architecture, message lifecycle, and test commands

[lovasoa]

Motivation

• Clarify and standardize the repository contributing guide so new contributors can find relevant code and procedures faster.
• Make the message lifecycle and socket/board flow explicit to reduce ambiguity when changing message shapes or tools.
• Surface test and formatting expectations and add a required-upkeep notice so documentation stays accurate when protocols or paths change.

Description

• Rewrote AGENTS.md with reorganized sections (baseline, architecture, message lifecycle, where to look by concern, test commands, formatting, change strategy, and required upkeep).
• Replaced terse file lists with descriptive pointers to key modules such as server/server.js, server/sockets.js, server/socket_policy.js, server/message_validation.js, server/boardData.js, client-data/js/board.js, and client-data/tools/.
• Added a detailed message lifecycle flow covering Tools.drawAndSend, Tools.send, buffered writes, server broadcast handling, normalizeIncomingMessage, board.processMessage, and client handleMessage behavior.
• Clarified test commands and formatting guidance and added an explicit note to update this file when behavior, paths, protocol shape, test commands, or architecture change.

Testing

• No unit or integration test changes were required for this documentation-only update.
• Ran npm run prettier/prettier-check to validate formatting and it completed successfully.
• The repository full test gate (npm test) remains the authoritative CI check but was not rerun for this doc change.

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Codex Task