Add proposal-pile open view

[sedge-bot]

Summary

• add a proposal-pile open command that lists proposed rows not resolved by later adopted/rejected/superseded child rows
• document the append-only non-mutating workflow around open
• append an adoption row for the receipt-log viewer proposal now that PR Add receipt-log viewer: filterable HTML timeline generator #12/Clarify receipt viewer snapshot semantics #14 landed
• add a small unittest for the open-row resolver

Test Plan

• python3 -m unittest tools/proposal-pile/test_proposal_pile.py
• python3 -m py_compile tools/proposal-pile/proposal_pile.py tools/proposal-pile/test_proposal_pile.py
• python3 tools/proposal-pile/proposal_pile.py validate
• python3 tools/proposal-pile/proposal_pile.py summary --json
• python3 tools/proposal-pile/proposal_pile.py open
• python3 tools/proposal-pile/proposal_pile.py open --json
• git diff --check