Add spreadsheet completeness stop guard

[EdbertChan]

Summary\n\nSpreadsheet work now has a completion guard that stops the workflow when source-backed entity coverage has not passed. It asks for bounded retries instead of allowing an incomplete workbook to be reported as finished.\n\n## Review Claim\n\nThe engine hook blocks spreadsheet completion claims without an explicit coverage receipt and supports at most three retry attempts.\n\n## Review Lane\n\nbehavior\n\n## Review Unit\n\nengine-runtime\n\n## Safety Invariant\n\nA spreadsheet completion claim cannot pass without the exact coverage receipt; incomplete coverage remains blocked after the retry limit.\n\n## Slice Rationale\n\nThis PR contains the engine hook and harness manifests. The separate product-skill PR contains schema and derived-estimate validation.\n\n## Non-goals\n\n- No changes to Google Sheet contents.\n- No automatic invention of competitor values.\n- No merge or deployment.\n\n

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Test Plan

\n\n- [x] python3 -m unittest discover -s engine/hooks/spreadsheet-completeness-guard/tests -p test_*.py\n- [x] positive and negative hook fixtures are included\n\n

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Revert Plan

\n\n- Safe to revert? Yes.\n- Revert command: git revert \n- Post-revert steps: Remove the hook registration if a partial revert leaves it enabled.\n- Data migration? No.\n\n

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Note

Medium Risk
New stop-hook enforcement can block agent turns on regex-matched completion language; behavior is bounded by the PASS receipt and tests, but false positives would affect workflow, not security or data.

Overview
Adds a new stop hook spreadsheet-completeness-guard (registered in hooks.toml) with Claude, Codex, and Cursor Stop manifests. On stop, it blocks assistant messages that claim spreadsheet/workbook completion unless the text includes SPREADSHEET_COVERAGE: PASS. Missing coverage prompts up to three source-collection retries via spreadsheet_retry_count; SPREADSHEET_COVERAGE: FAIL after that limit keeps the stop with guidance to list unresolved entity/period keys instead of declaring done.

Also introduces the spreadsheet-authoring product skill: raw→aggregation→chart conventions, typed period columns vs observation points, and validate_schema.py for CSV fixtures (required columns, no qualifiers in period fields, unique observation keys), plus skill fire/silent fixtures and unit tests for the hook and validator.

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[EdbertChan]

Replaced by #1221. This branch also carried the spreadsheet-authoring skill, so the engine-runtime review unit could not pass. #1221 is only the stop hook.