refactor(memory): migration files + embed.FS

- shell/memory/migrations/001_init.sql extraido del schema inline
- sqlite.go: applyMigrations() con embed.FS aplicado al abrir
- aplica regla db_migrations.md (fn_registry/.claude/rules/)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

shell/memory/migrations/001_init.sql

@@ -0,0 +1,22 @@
+-- Memoria persistente del agente: facts (clave/valor por sujeto) + messages (historial chat).
+
+CREATE TABLE IF NOT EXISTS facts (
+    agent_id   TEXT NOT NULL,
+    subject    TEXT NOT NULL,
+    key        TEXT NOT NULL,
+    value      TEXT NOT NULL,
+    updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (agent_id, subject, key)
+);
+
+CREATE TABLE IF NOT EXISTS messages (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    agent_id   TEXT NOT NULL,
+    room_id    TEXT NOT NULL,
+    role       TEXT NOT NULL,
+    content    TEXT NOT NULL,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_messages_room ON messages(agent_id, room_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_facts_subject ON facts(agent_id, subject);