docs: add schema-migrations reference for SQLite chain and legacy.

Centralises migration history, 1.x rules, and the 2.x squash gate so
roadmap and architecture no longer carry a stale v5 snapshot.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/development.md

@@ -424,6 +424,7 @@ There is no `docs/archive/` directory.
 | As-built design | [architecture.md](architecture.md) |
 | Development process (this file) | [development.md](development.md) |
 | Security requirements and accepted risks | [security.md](security.md) |
+| SQLite schema and migrations | [schema-migrations.md](schema-migrations.md) |
 | Roadmap (1.x+) | [roadmap.md](roadmap.md) |
 | Active design plans | [plans/](plans/) |
 | Release history | [CHANGELOG.md](../CHANGELOG.md) |
@@ -453,6 +454,8 @@ Out of scope for v1.x: `CONTRIBUTING.md`, man pages, a separate docs site
    fails on an undocumented `loadConfig` or build-script key.
 3. **New gaps** go into [roadmap.md](roadmap.md) (or the active plan file), not
    silent drive-by edits.
+4. **Schema changes:** update [schema-migrations.md](schema-migrations.md) in the
+   same change as a new `internal/store/migrations/*.sql` file.
 
 ### Verifying docs against code