feat(soul): add sessions layer, MCP connectors, conversation continuity fix

- sessions.el: new sessions module with session management and approval gate
- routes.el: wire /api/sessions routes (list, get, create, approve, tool_result)
- chat.el: thread-aware activation — short messages anchor to last reply
  before engram compilation so follow-ups stay on-topic
- chat.el: agentic path tracks per-session history (session_hist_{id})
  instead of shared conv_history, seeding each turn with prior context
- chat.el: add call_neuron_mcp, dispatch_tool, is_builtin_tool, next_bridge_id
  agentic_loop, bridge_save, agentic_resume, handle_tool_result
- dist/soul: rebuild with all of the above

sessions.elh

@@ -0,0 +1,14 @@
+// auto-generated by elc --emit-header — do not edit
+extern fn session_title_from_message(message: String) -> String
+extern fn session_make_content(id: String, title: String, created_at: Int, updated_at: Int) -> String
+extern fn session_create(body: String) -> String
+extern fn session_list() -> String
+extern fn session_get(session_id: String) -> String
+extern fn session_delete(session_id: String) -> String
+extern fn session_update_title(session_id: String, body: String) -> String
+extern fn session_search(query: String) -> String
+extern fn session_hist_load(session_id: String) -> String
+extern fn session_hist_save(session_id: String, hist: String) -> Void
+extern fn session_update_meta_timestamp(session_id: String) -> Void
+extern fn session_auto_title(session_id: String, first_message: String) -> Void
+extern fn handle_session_approve(session_id: String, body: String) -> String