feat(palier-3): recherche globale, mes issues, briefing ntfy, MCP lecture

- client: search_issues (/repos/issues/search, type=issues,
  assigned_by), delete_label + forget_label_cache; models: kv_store
- /search groupe par projet avec colonne Kanban; /my = issues
  ouvertes assignées à l'utilisateur du jeton (get_user)
- briefing: _briefing_data (focus, âgées, milestones <= 7 j,
  activité journal), page /briefing, /api/briefing, envoi ntfy
  manuel + planificateur lifespan à KANBANTE_BRIEFING_TIME
  (kv briefing_last_sent, pas de doublon); app/briefing.py =
  format texte + envoi ntfy (en-têtes ASCII)
- MCP: POST /mcp JSON-RPC (initialize, tools/list, tools/call,
  ping, notifications 202), 7 outils en lecture seule, GET /mcp
  de découverte; erreurs -32601/-32700, isError pour les outils
- focus refactoré en _collect_focus_groups réutilisé par le
  briefing et l'outil MCP get_focus

app/models.py

@@ -79,6 +79,11 @@ def init_db() -> None:
             CREATE INDEX IF NOT EXISTS idx_board_repo
                 ON board_state(repo_full_name, column_name);
 
+            CREATE TABLE IF NOT EXISTS kv_store (
+                key TEXT PRIMARY KEY,
+                value TEXT NOT NULL
+            );
+
             CREATE INDEX IF NOT EXISTS idx_events_repo
                 ON board_events(repo_full_name, issue_number, created_at);
             """
@@ -234,6 +239,23 @@ def reconcile_board_state(
     return changed
 
 
+def get_kv(key: str) -> str | None:
+    with get_db() as conn:
+        row = conn.execute(
+            "SELECT value FROM kv_store WHERE key = ?", (key,)
+        ).fetchone()
+    return row["value"] if row else None
+
+
+def set_kv(key: str, value: str) -> None:
+    with get_db() as conn:
+        conn.execute(
+            "INSERT INTO kv_store (key, value) VALUES (?, ?) "
+            "ON CONFLICT(key) DO UPDATE SET value = excluded.value",
+            (key, value),
+        )
+
+
 # ── Event journal ──────────────────────────────────────────
 
 EVENT_TYPES = ("created", "moved", "closed", "reopened", "commented")