chore: add Exchange header naming conventions design document

[gnodet]

Summary

Add design/headers.adoc documenting the Exchange header naming convention that the ongoing alignment work (CAMEL-2357x/CAMEL-2359x by @oscerd) is enforcing.

Depends on #23447 (rename proposals/ to design/).

The convention document covers:

• Naming rule: "Camel" + ComponentName + Feature in PascalCase
• Java constant patterns and @Metadata annotations
• Protocol header exemptions (e.g., Swift X-Container-Meta-, HTTP Content-Type)
• Security interaction with HeaderFilterStrategy — why the Camel* prefix matters
• Upgrade guide requirements for header renames
• Backward compatibility policy (hard break, no aliases, as of 4.21)
• Backport policy for maintenance branches

Test plan

• Verify design/headers.adoc exists with correct frontmatter
• Verify AGENTS.md references the new document
• No build impact (non-compiled files only)

Claude Code on behalf of Guillaume Nodet

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