feat(db): migration 007 — conversations.space_id + seed companion agent

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/migrations/007_companion.sql

@@ -0,0 +1,14 @@
+-- lib/db/migrations/007_companion.sql
+-- Plan 5: per-Space ambient companion conversation + default companion agent.
+
+ALTER TABLE conversations
+  ADD COLUMN space_id uuid REFERENCES spaces(id) ON DELETE CASCADE;
+
+CREATE INDEX idx_conversations_space ON conversations(space_id, started_at DESC);
+
+INSERT INTO agents (slug, name, kind, model, capabilities)
+VALUES (
+  'companion', 'Companion', 'claude', NULL,
+  '{"read":true,"suggest":true,"write":false}'::jsonb
+)
+ON CONFLICT (slug) DO NOTHING;