Harden SQLite store logs, artifacts, and lifecycle

[vamgan]

Summary

• append log chunks directly instead of read/modify/write rewriting the full log file
• prevent artifact file path traversal outside each task artifact directory
• add an explicit close() API for the SQLite connection
• cover log append, artifact path validation, and close/reopen behavior in tests

Tests

• npm ci
• npm run typecheck
• npm test -- --run
• npm run build

Fixes #2
Fixes #3
Fixes #4