feat(dashboard): streamlit skeleton + Overview page + data layer

Aggiunge scheletro multipage Streamlit per Phase 1:
- modulo data.py con helper (list_runs_df, get_run_overview,
  generations_df, evaluations_df, genomes_df) sopra Repository.
- streamlit_app.py entry point con DB_PATH da env.
- pages/01_overview.py per elenco run + metriche + config JSON.
- smoke test import di multi_swarm.dashboard.data.

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

src/multi_swarm/dashboard/pages/01_overview.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import streamlit as st
+
+from multi_swarm.dashboard.data import get_repo, get_run_overview, list_runs_df
+
+st.title("Overview")
+
+db_path = st.session_state.get("db_path", "./runs.db")
+repo = get_repo(db_path)
+
+runs = list_runs_df(repo)
+if runs.empty:
+    st.info("Nessuna run nel database. Esegui `scripts/run_phase1.py` per generarne una.")
+    st.stop()
+
+st.subheader("Tutte le run")
+st.dataframe(runs[["id", "name", "started_at", "completed_at", "status", "total_cost_usd"]])
+
+selected = st.selectbox("Seleziona run per dettaglio", runs["id"].tolist())
+overview = get_run_overview(repo, selected)
+
+col1, col2, col3, col4 = st.columns(4)
+col1.metric("Status", overview["status"])
+col2.metric("Cost (USD)", f"{overview['total_cost_usd']:.4f}")
+col3.metric("Started", overview["started_at"])
+col4.metric("Completed", overview["completed_at"] or "—")
+
+st.subheader("Config")
+st.json(overview["config"])