Heads-up: SchemaBot's core will become database-agnostic (PostgreSQL support planned)

## What

A heads-up for anyone following or building on SchemaBot: we intend to make the
codebase **database-agnostic**, so new database families — **PostgreSQL first** —
can be added as implementations rather than rewrites.

We're posting this for **transparency**: this is a direction we've decided on,
not an open question. There's no near-term behavior change, and existing
MySQL/Vitess deployments are unaffected — the point is to set expectations now.

## Two independent dimensions

The MySQL coupling has two dimensions that are often conflated. They're tracked
together but ship separately:

- **Target databases** (the databases SchemaBot runs schema changes *against*).
This is the user-visible goal. The engine layer is already pluggable (Spirit
for MySQL, PlanetScale for Vitess, and a Postgres engine stub already exists);
the remaining work is DDL parsing, dialect assumptions, and the Postgres
online-DDL mechanism.
- **SchemaBot's own state store** (plans, applies, tasks, checks, leases, etc.).
This lives behind a clean storage interface today with a single MySQL
implementation. Making it dialect-agnostic is independent of target support
and can trail it.

## Guiding principles

- **Second implementation before abstraction polish** — the interfaces already
exist; we won't redesign them until a real Postgres implementation exposes
pressure points.
- **Capability gates, not lowest-common-denominator** — MySQL-only features stay
available for MySQL and are gated by engine capability interfaces, not removed.
- **No behavior change for existing MySQL/Vitess deployments** at any merge
point — every step is dormant or dialect-routed.

## The hard part

The Postgres online-DDL mechanism is the long pole — it shapes the engine's
cutover/revert state machine and needs its own mini-design before implementation.
Advisory-lock semantics also differ enough from MySQL's named locks to warrant a
deliberate design rather than a direct translation.

## Scope

- PostgreSQL is the first new target; the architecture is meant to generalize
beyond it.
- MySQL and Vitess remain first-class, fully supported.

Start by reviewing the pluggable engine layer, the existing PostgreSQL engine stub, and the storage interface described in the issue. The PostgreSQL online-DDL mechanism and advisory-lock semantics need a mini-design before implementation; done is not defined as a specific code change in this issue.

Written by the indexing model from the issue text.