Add background task management guidelines to CLAUDE.md

[codekiln]

Add warning about never using tail -f to monitor background tasks, as it leaves processes running indefinitely.

Changes needed:

• Add section about background task management to CLAUDE.md
• Document correct approach using TaskOutput tool with block=true
• Add warning about tail -f running forever

Context:
This issue came up twice where tail -f was left running in background, requiring manual cleanup.