CAMEL-24228: Clarify upgrade guide is for migration only, not new features

[gnodet]

Claude Code on behalf of gnodet

Summary

Fixes the overly broad wording in the Code Quality section of AGENTS.md (and CLAUDE.md which symlinks to it) that stated "Any user-visible change must be documented in the upgrade guide." This incorrectly suggested that new features should be added to the upgrade guide.

What changed

• Before: "Any user-visible change must be documented in the upgrade guide (...) and in the relevant command or component documentation page."
• After: Separates the guidance into two clear directives:
  1. New features → document in the relevant component/command docs page
  2. Migration-affecting changes (changed defaults, removed/renamed options, API/SPI changes, deprecations, etc.) → document in the upgrade guide
• Adds an explicit statement: "The upgrade guide is for migration only — do NOT add new features to it."

What stays the same

• The list of migration-relevant changes (changed defaults, renamed options, API changes, etc.) is preserved
• The backport guidance ("upgrade guide entry must be on main branch") is unchanged
• The Deprecation section (line 552) correctly references the upgrade guide and is unchanged
• The Security checklist (line 474) correctly references upgrade-guide for relaxed defaults and is unchanged

Test plan

• Verified CLAUDE.md is a symlink to AGENTS.md — only one file needs editing
• Verified the diff only touches the Code Quality section
• Cross-checked the remaining upgrade-guide references (Deprecation section, Security checklist) are correct and untouched
• Verified the change aligns with the actual upgrade guide content (camel-4x-upgrade-guide-4_9.adoc documents API changes, renamed constants, changed behavior — not new features)

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