Move migrations to internal/db/migrations (closes #96)

REPO_POLICIES.md puts migrations in internal/db/migrations/ as
000_migration.sql and 001_schema.sql. The two files move there with
their contents unchanged. go:embed cannot reach outside its own
package, so internal/db/migrations has a small package that embeds
them, and internal/database reads them through its FS(). The database
package stays where CONVENTIONS.md puts it; moving it would change
existing test files in other packages.

The version still comes from the filename prefix, so a database that
has recorded versions 0 and 1 runs neither again. A new test applies
the migrations twice to one database file and checks that the second
run applies nothing.

Model: opus-5-5

internal/db/migrations/000_migration.sql

@@ -0,0 +1,9 @@
+-- Migration 000: Schema migrations tracking table
+-- Applied as a bootstrap step before the normal migration loop.
+
+CREATE TABLE IF NOT EXISTS schema_migrations (
+    version INTEGER PRIMARY KEY,
+    applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+INSERT OR IGNORE INTO schema_migrations (version) VALUES (0);