Document that migrations are supported and none are added before 1.0 (closes #68)

#146 called the numbered schema
files a bootstrap and said vaultik has no upgrade path between versions.
That was wrong: they are migrations, and database.New applies any the
database has not recorded. None are added before 1.0 because nothing is
installed anywhere yet; after 1.0 each schema change is a new numbered
file and the local database is migrated on update.

docs/DATAMODEL.md gets a Schema Migrations section that owns the
explanation. The README caveat and roadmap entry and AGENTS.md policy 13
say the same and link to it. CLAUDE.md, the owner's file, gets a
one-line edit so it no longer says migrations are not needed.

Model: opus-5-5

AGENTS.md

@@ -102,14 +102,12 @@ Version: 2025-06-08
     build files are acceptable in the root, but source code and other files
     should be organized in appropriate subdirectories.
 
-13. Pre-1.0: NEVER write database migrations. There are no live databases
-    anywhere — every user's local index can be rebuilt from a fresh full
-    backup. To change the schema, edit `internal/database/schema/001.sql`
-    (and any code that touches the affected tables) directly; do not add new
-    numbered schema files. Those numbered files and the `schema_migrations`
-    table they populate only bootstrap a fresh database — they are not an
-    upgrade path. The local index is disposable until 1.0 ships and is
-    tagged; once 1.0 is tagged that clause expires and the question of
-    upgrading existing indexes returns. See [`docs/DATAMODEL.md`](docs/DATAMODEL.md)
-    for the full explanation.
+13. Pre-1.0: NEVER add a database migration. Migrations are supported, but
+    nothing is installed anywhere yet, so there is nothing to migrate. To
+    change the schema, edit `internal/database/schema/001.sql` (and any
+    code that touches the affected tables) directly. After 1.0, each schema
+    change is a new numbered file in that directory and a released file is
+    never edited; an existing local database is then migrated when vaultik
+    is updated. See
+    [`docs/DATAMODEL.md`](docs/DATAMODEL.md#schema-migrations).