docs(FN-7952): document completed PostgreSQL cutover

[gsxdsm]

Summary

Operators and contributors now have one current contract for the completed PostgreSQL cutover: PostgreSQL is mandatory runtime authority, legacy SQLite files are read-only migration evidence, and rollback is restore-based rather than a fallback toggle. The major changeset communicates the same boundary to published CLI users.

Operational guidance

• Documents the authoritative storage inventory and the five intentional legacy SQLite readers.
• Defines maintenance-window migration, backup, restore-test, ownership, and isolation checks.
• Clarifies lifecycle ownership for CLI, desktop, dashboard, engine, and bundled plugins.
• Supersedes the earlier readiness review without deleting its historical record.

Validation

• Changeset format check passes.
• pnpm test:gate passes all 478 gate tests on the completed stack.
• Every file from the original oversized PR is represented in the replacement stack; this PR changes 22 files.

Stack

• Depends on fix(FN-7952): migrate bundled plugins to PostgreSQL #2111 → fix(FN-7952): require PostgreSQL in CLI and desktop #2110 → fix(FN-7952): cut runtime services over to PostgreSQL #2109 → fix(FN-7952): establish PostgreSQL core authority #2108.
• Merge bottom-up in that order.

Related: #2105

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/postgres-cutover-docs

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[greptile-apps]

Greptile Summary

This PR documents PostgreSQL as Fusion's runtime storage authority. The main changes are:

• Defines legacy SQLite files as read-only migration evidence.
• Documents migration, backup, restore, ownership, and isolation procedures.
• Updates CLI, dashboard, engine, plugin, and storage guidance for the completed cutover.
• Adds a major changeset for the published CLI package.

Confidence Score: 5/5

This looks safe to merge.

• The updated health documentation resolves the previously reported operator-facing mismatch.
• No blocking issue remains in the latest documentation changes.

Important Files Changed

Filename	Overview
docs/architecture.md	Updates the architecture, persistence, health, and transaction contracts for PostgreSQL.
docs/PLUGIN_AUTHORING.md	Documents declarative PostgreSQL schema setup and the legacy SQLite hook boundary.
docs/storage.md	Defines PostgreSQL runtime authority and the limited migration role of legacy SQLite data.
docs/postgres-migration-review-2026-07-14.md	Records the current cutover inventory, operational contract, and verification status.

_{Reviews (3): Last reviewed commit: "Merge branch 'feature/postgres-cutover-p..." | Re-trigger Greptile}