Preserve history after incomplete agent scans

[VasiHemanth]

Summary

• Prevent unreadable Codex rollouts from replacing durable token rollups with empty data (Codex: exception on unreadable rollout file clears stub flag, zero-overwrites persisted session #221).
• Preserve Cline and Hermes history presence when their SQLite or VS Code sources cannot be read (Cline: bare except around SQLite query can zero all sessions, then mark_absent prunes them from history #222, Hermes: bare except around per-DB scan loop drops all sessions for that profile, prunes them from history #223).
• Add backend and frontend test CI for pull requests and pushes (Tests exist but no CI runs them — add GitHub Actions test workflow #65).
• Pin the frontend test runner, update Next.js to 16.3.0, and refresh the lockfile so the dependency audit is clean.

Validation

• python -m pytest backend -q (467 passed)
• npm ci && npm test (23 passed)
• npx tsc --noEmit
• npm run build
• npm audit --audit-level=high (0 vulnerabilities)

Closes #221
Closes #222
Closes #223
Closes #65