docs(object-storage): add S3 conditional writes guide (EN + FR)

- Add guide.en-gb.md and guide.fr-fr.md covering If-Match and If-None-Match
  conditional headers for PutObject, DeleteObject, GetObject, HeadObject
  and CompleteMultipartUpload
- Add meta.yaml with guide UUID and slug
- Update .gitignore to exclude agent and instruction files

Author: Laurent Song

.gitignore

@@ -14,4 +14,8 @@ llm-config/
 cp-extraction
 CLAUDE.md
 CLAUDE.local.md
-.mcp.json
+.mcp.json
+.github/agents/doc-writer.agent.md
+.github/instructions/object-storage-docs.instructions.md
+.github/agents/analyst.agent.md
+.github/agents/output/