chore: Renaming updates

[shahar-davidson]

This PR primarily completes a sai → agentsonar rename and improves developer/operator ergonomics.

• Rebranded CLI and config references across docs and examples (docs/commands.md, docs/classifiers.md, examples/siem_forward/README.md), including env var/path updates.
• Expanded README.md with a clearer “what it does/how it works” section and practical usage guidance.
• Updated docs/development.md with corrected setup steps (cd agentsonar), clearer build/test workflow, and make setup-bpf guidance.
• Fixed daemon startup reliability in cmd/agentsonar/main.go by creating the log directory before opening the log file.
• Hardened Makefile cleanup/build paths by anchoring BUILD_DIR to the Makefile’s directory (avoids cleaning/building in the wrong bin/).
• Broadened .gitignore to cover common build artifacts, OS/editor files, logs, and temp outputs.