feat: move schema_migrations table creation into 000_bootstrap.sql

Move the inline CREATE TABLE IF NOT EXISTS schema_migrations from Go
code into a dedicated 000_bootstrap.sql file, following the sneak/pixa
pattern. The bootstrap SQL is executed directly before the migration
loop, which now starts from 001+. The bootstrap file also handles its
own INSERT OR IGNORE so the Go code does zero inserts for version 0.

closes #91

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/schema/000_bootstrap.sql

@@ -0,0 +1,6 @@
+-- Bootstrap: create the schema_migrations table itself.
+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);