multi-agent research: parallel LLM-powered investigation

new research tool spawns 3-25 micro-agents (ministral-3b) in
parallel via futures::join_all. each agent gets its own Mistral
conversation with full tool access.

recursive spawning up to depth 4 — agents can spawn sub-agents.
research sessions persisted in SQLite (survive reboots).
thread UX: 🔍 reaction, per-agent progress posts, ✅ when done.

cost: ~$0.03 per research task (20 micro-agents on ministral-3b).

Cargo.lock

@@ -4184,6 +4184,7 @@ dependencies = [
  "deno_ast",
  "deno_core",
  "deno_error",
+ "futures",
  "libsqlite3-sys",
  "matrix-sdk",
  "mistralai-client",