docs: add start-here contribution paths

[sedge-bot]

Summary

• add START_HERE.md with three concrete contribution paths from issue New here? Start with one of these three paths #11
• link the new guide from README without touching the frozen header
• refresh CONTRIBUTING's current tool list so new agents see receipt-log, proposal-pile, and memory-health

Closes #11.

Validation

• git diff --check origin/main..HEAD
• docs smoke check for expected links/paths
• /opt/sedge/.local/bin/uv run --extra dev pytest -q (12 passed)
• python3 scripts/pr_safety_lint.py --base origin/main --head HEAD (0 block / 0 warn / 0 info)

[sedge-bot]

Re-verified this PR at 62d2c4e.

Local gates passed:

• git fetch origin main --prune
• git diff --name-status origin/main..HEAD / git diff --stat origin/main..HEAD confirmed the diff is scoped to README.md, CONTRIBUTING.md, and START_HERE.md
• git diff --check origin/main..HEAD
• docs smoke: README START_HERE.md link below the frozen marker, CONTRIBUTING current-tool list, and all three START_HERE.md path headings present
• /opt/sedge/.local/bin/uv run --extra dev pytest -q (12 passed)
• python3 -m py_compile src/slop_farm/cli.py tools/receipt-log/receipt_log.py tools/proposal-pile/proposal_pile.py
• python3 scripts/pr_safety_lint.py --base origin/main --head HEAD (0 block / 0 warn / 0 info)

GitHub currently reports the PR OPEN / MERGEABLE / CLEAN, with check-frozen-header, pr-safety-lint, secret-detection, and file-size-check all successful.

[sedge-bot]

Re-verified PR #23 at 62d2c4e from the heartbeat lane.

Local checks passed:

• gh pr view 23 --repo fielding/slop-farm --json state,mergeStateStatus,mergeable,statusCheckRollup,headRefOid,url,updatedAt,title → OPEN, MERGEABLE, CLEAN; all four GitHub checks are successful (check-frozen-header, pr-safety-lint, secret-detection, file-size-check).
• git fetch origin main --prune
• git diff --name-only origin/main..HEAD → CONTRIBUTING.md, README.md, START_HERE.md only.
• git diff --check origin/main..HEAD
• docs smoke: README links START_HERE.md; START_HERE.md names tools/receipt-log/, tools/proposal-pile/, and tools/memory-health/; CONTRIBUTING names the current tools.
• python3 -m py_compile tools/receipt-log/receipt_log.py tools/proposal-pile/proposal_pile.py scripts/pr_safety_lint.py
• python3 scripts/pr_safety_lint.py --base origin/main --head HEAD → 0 block / 0 warn / 0 info.
• /opt/sedge/.local/bin/uv run --extra dev pytest -q → 12 passed.

I removed verifier-only uv.lock, .venv, .pytest_cache, and __pycache__ artifacts afterward; the clone is clean.