docs(#4887): rework agent guides and add CLAUDE.md

[tenthe]

Purpose

Overview

This PR reworks the repository's agent instruction files so that they carry the facts an AI coding agent cannot derive from the code, and adds a CLAUDE.md so Claude Code loads them. The root guide now states the toolchain, build and CI contract; the module guides gain concrete recipes; duplicated content across the UI and design docs is consolidated with an explicit precedence order.

Changes

• Adds CLAUDE.md, which imports AGENTS.md and explains that nested guides must be read before editing in their directories.
• Rewrites the root AGENTS.md around the toolchain (JDK 25 build, Java 17 language level), the fast test loop versus the root verify that alone runs the license check, code standards, commit conventions and an index of all nested guides.
• Moves the security-researcher procedure out of the root guide into a security-triage skill under .claude/skills/, so it loads only for vulnerability work.
• Adds recipes to the module guides: adding a migration (service-core), an endpoint (rest), an extension (extensions) and a storage type (storage-couchdb); removes the per-module validation sections now covered by the root.
• Adds guides for the Python and Go clients, covering their toolchains and CI gates.
• Cross-links DESIGN.md, PRODUCT.md, ui/STYLEGUIDE.md and ui/AGENTS.md with a stated hierarchy (DESIGN.md is normative for tokens and rules), corrects the UI component list and layout package name, and removes verbatim duplicates.
• Exempts CLAUDE.md and .claude/** from the RAT license check, matching the existing AGENTS.md rule.

Remarks

PR introduces (a) breaking change(s): yes

PR introduces (a) deprecation(s): yes