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

The docs now say vaultik supports migrations. The numbered files in `internal/database/schema/` are migrations: `schema_migrations` records which have run, and opening a database applies any that have not. None are added before 1.0 because nothing is installed anywhere yet, so a schema change edits `001.sql` directly. After 1.0 each change is a new numbered file, and an existing local database is migrated when vaultik is updated.

`docs/DATAMODEL.md` owns the explanation. The README caveat and roadmap entry and `AGENTS.md` policy 13 link to it. This replaces the wording from #146, which said there was no upgrade path.

Disclosure: `CLAUDE.md` line 33, the owner's file, changes from "do not need to support migrations" to "do not add migrations before 1.0".

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).