rust: nexus-core storage, vault, keychain, export/import, data-dir migration

rust/core/migrations/003_snippets.sql

@@ -0,0 +1,15 @@
+-- Migration 003: Add snippets table for the command snippet library.
+-- Stores reusable commands that can be sent to active sessions.
+
+CREATE TABLE IF NOT EXISTS snippets (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    command TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    folder TEXT NOT NULL DEFAULT '',
+    tags TEXT NOT NULL DEFAULT '[]',
+    created_at TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+INSERT INTO schema_version (version, applied_at) VALUES (3, datetime('now'));