fix(tuttid): finish workspace write pool migration

[chovy-ai]

Summary

• use the SQLite writer pool for the remaining workbench migration transaction
• update migration fixtures to write through the writer pool

Root cause

PR #1236 replaced SQLiteStore.db with separate writeDB and readDB pools, but the workbench target-identity migration and two test fixtures retained references to the removed db field. This makes current main fail Go compilation, including unrelated PRs rebased onto it.

Fix Scope Justification

• Root cause: the first incorrect state is at the workspace SQLite storage boundary, where three remaining write operations reference a field removed by the read/write pool split.
• Why this cannot be fixed at a lower layer: database/sql and SQLite are functioning correctly; the invalid field references are in Tutti-owned migration code. Mapping these operations to the existing writeDB is the smallest owned-layer correction and preserves the pool architecture introduced by fix(tuttid): separate SQLite read and write pools #1236.

Validation

• go test ./services/tuttid/data/workspace/...
• git diff --check

The full Go workspace runner was also attempted without the generated builtin app artifact and failed only because generated/tutti-onboarding/tutti-onboarding-0.1.0.zip was absent; the affected package passed.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.