docs: add epoch ms unit comment to created_at columns

[G0-0000]

created_at uses Date.now() (epoch ms), but schema has no unit comment. This PR adds -- epoch ms (Date.now()) comments to all created_at columns for clarity.

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Summary by cubic

Adds -- epoch ms (Date.now()) comments to all created_at columns in the schema to document the timestamp unit, with no behavior change.

^{Written for commit 6e3751e. Summary will update on new commits.}

Greptile Summary

This PR documents the unit used by created_at columns in the migration and database-initialization schemas.

• Adds inline epoch-millisecond annotations to migration DDL.
• Adds matching annotations to the current initialization schema.
• Does not alter schema structure or runtime behavior.

Confidence Score: 5/5

The PR appears safe to merge because it only adds valid SQL comments and introduces no behavioral changes.

The added comments preserve SQL parsing, match the timestamp unit used by current production writers, and do not change database schemas or data paths.

Important Files Changed

Filename	Overview
packages/plugin/src/features/magic-context/migrations.ts	Adds epoch-millisecond comments to historical migration definitions without changing executable schema semantics.
packages/plugin/src/features/magic-context/storage-db.ts	Adds matching timestamp-unit comments to database initialization DDL without changing schema behavior.

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[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[magic-alfonso]

Thanks — the intent is right (the unit is not written down anywhere a reader of the schema would find it), but I am going to decline this shape rather than merge it, for two reasons.

Applied migrations are history. SQLite stores the CREATE statement text verbatim in sqlite_master.sql, comments included, so editing the SQL of a migration that has already run means a database migrated last month and one migrated today carry different sqlite_master text for the same table. Nothing breaks at runtime, but every schema-equivalence check we run between a fresh install and a replayed upgrade now has to know to ignore that, and the comment never reaches the databases that already exist. Shipped migrations are append-only in this repo; only the fresh-install schema in storage-db.ts and new migrations are editable.

The unit is not uniform enough to assert blanket. We audited several created_at writers this week (historian-promoted facts and memory-mirror rows among them) and found columns that had been written as 0, and paths where the Rust module and the TypeScript host disagreed on the stamp until fixed. A comment that says epoch ms (Date.now()) on 31 columns is a claim about 31 writers; without checking each one it can mislead the next reader more than silence would.

If you want to carry this forward, the shape that would merge: a short table in the storage section of ARCHITECTURE.md (or a docs/ note) listing the timestamp columns with their unit and the writer that stamps them, each entry verified against the writer — and, for fresh installs only, the same comment on the storage-db.ts schema where it is actually verified. Happy to review that.

[G0-0000]

Reshaped per maintainer feedback: timestamp units table (epoch ms, per-column verified writers) added to ARCHITECTURE.md, and -- epoch ms (Date.now()) comments added to the fresh-install schema only — no migration DDL touched. All code changes from the original PR shape removed; new PR: #458 (#458).