Reconcile the schema/migration docs with the code (closes #68)

Four documents told different stories about the database schema. docs/DATAMODEL.md now owns the explanation and separates two things: the policy, which is unchanged (no supported upgrade path between versions; delete the local index with vaultik database delete and back up again), and the schema bootstrap that does exist (numbered files in internal/database/schema applied to a fresh database and recorded in schema_migrations).

README.md and AGENTS.md are reworded to match and link there. AGENTS.md names the real file to edit, internal/database/schema/001.sql, and notes that the pre-1.0 disposability clause expires on tagging. No code changed.

Judgement call: REPO_POLICIES.md still names a different schema file; it is cross-project policy and was left alone.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

AGENTS.md

@@ -104,7 +104,12 @@ Version: 2025-06-08
 
 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. When the schema changes, just change `schema.sql` (and any code
-    that touches the affected tables). The local index is disposable until
-    1.0 ships and is tagged.
+    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.
 

README.md

@@ -457,9 +457,13 @@ Key fields:
   sequentially. Restore speed is bound by single-stream throughput.
 * **Device nodes, named pipes, and sockets are silently skipped.** Only
   regular files, directories, and symlinks are backed up.
-* **No database migrations.** If the local SQLite schema changes between
-  versions, delete the local database (`vaultik database delete`) and run
-  a full backup. Remote storage is unaffected.
+* **No upgrade path between versions.** There is no supported way to carry
+  an existing local index across a schema change; if the local SQLite
+  schema changes between versions, delete the local database (`vaultik
+  database delete`) and run a full backup. Remote storage is unaffected.
+  (The binary does embed numbered schema files and a `schema_migrations`
+  table to bootstrap a fresh database — see [`docs/DATAMODEL.md`](docs/DATAMODEL.md)
+  — but that is not an upgrade path.)
 * **Files that change during backup may be inconsistent.** There is no
   filesystem snapshot or freeze. If a file is modified between the scan
   and chunk phases, the backed-up copy may reflect a partial write.
@@ -529,10 +533,12 @@ priority.
   another host" workflow works but isn't documented as a
   first-class operation in this README. Worth a dedicated section
   once it's settled.
-* **Schema migrations.** Currently nonexistent — pre-1.0 schema
-  changes are handled by `vaultik database delete` plus a full
-  re-scan. Post-1.0 we'll need a migration story to keep existing
-  index databases usable across upgrades.
+* **Cross-version schema upgrades.** There is no upgrade path between
+  released versions — pre-1.0 schema changes are handled by `vaultik
+  database delete` plus a full re-scan (see
+  [`docs/DATAMODEL.md`](docs/DATAMODEL.md)). Post-1.0 we'll need a
+  migration story to keep existing index databases usable across
+  upgrades.
 * **Storage backend coverage tests.** S3, file://, and rclone://
   all share the Storer interface but the rclone path is the least
   exercised in CI.

docs/DATAMODEL.md

@@ -5,11 +5,30 @@
 Vaultik uses a local SQLite database to track file metadata, chunk mappings, and blob associations during the backup process. This database serves as an index for incremental backups and enables efficient deduplication.
 
 **Important Notes:**
-- **No Migration Support (pre-1.0)**: Vaultik does not support database schema
-  migrations. The local index is treated as disposable — if the schema changes,
-  delete the local SQLite database (`vaultik database delete`) and run a full
-  backup. The remote storage is unaffected; the new index will re-deduplicate
-  against existing remote blobs.
+
+This section is the authoritative explanation of the schema/migration story;
+other documents (the README and `AGENTS.md`) link here.
+
+- **No upgrade path between versions (pre-1.0)**: Vaultik has no supported way to
+  carry an existing local index across a schema change. The index is disposable
+  — if the on-disk schema changes between versions, delete the local SQLite
+  database (`vaultik database delete`) and run a full backup. Remote storage is
+  unaffected; the new index re-deduplicates against existing remote blobs. This
+  is the standing project policy, and it is separate from the schema bootstrap
+  described next.
+- **Schema bootstrap**: a fresh database is populated from numbered SQL files
+  embedded in the binary under `internal/database/schema/`. `000.sql` creates the
+  `schema_migrations` table; `001.sql` creates the application tables. On opening
+  a database the code applies each numbered file that has not yet run and records
+  its version in `schema_migrations`. This bootstraps a new database; it does not
+  upgrade an existing one between released versions.
+- **Changing the schema (pre-1.0)**: edit `internal/database/schema/001.sql` (and
+  the code that touches the affected tables) directly. Do not add new numbered
+  files — there is no installed base to migrate.
+- **Disposability expires at 1.0**: the index is treated as disposable only until
+  1.0 ships and is tagged. Once 1.0 is tagged that clause expires and the
+  question of upgrading existing indexes returns. It is deliberately left open
+  here.
 - **Version Compatibility**: In rare cases, you may need to use the same version
   of Vaultik to restore a backup as was used to create it. This ensures
   compatibility with the metadata format stored in S3.