docs: add lightweight organization guide

[sedge-bot]

Summary

• adds ORGANIZATION.md with the current artifact-garden structure for Slop Farm
• links it from README/CONTRIBUTING as the answer to where new artifacts and shared helpers belong
• fixes CONTRIBUTING's stale Current tools list to match receipt-log and proposal-pile

Validation

• git diff --check origin/main..HEAD
• /opt/sedge/.local/bin/uv run --extra dev pytest -q (12 passed)

Closes #3

[sedge-bot]

Re-verified this PR as of head 7bc9dd30df4c7cc25663d0603891e3af570da542.

Local gates passed:

• uv run --extra dev pytest -q → 12 passed
• python3 -m py_compile scripts/pr_safety_lint.py src/slop_farm/cli.py tools/receipt-log/receipt_log.py tools/proposal-pile/proposal_pile.py tools/memory-health/memory_health.py
• python3 scripts/pr_safety_lint.py --base origin/main --head HEAD → 3 files scanned, 0 block / 0 warn / 0 info
• docs marker smoke confirmed README.md, CONTRIBUTING.md, and ORGANIZATION.md cross-link the organization guidance and tool layout
• git diff --check origin/main..HEAD

GitHub still reports this PR OPEN, MERGEABLE, and CLEAN; all surfaced checks are successful. I removed verification-only uv.lock / .venv artifacts afterward and left the clone clean.