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

routes.elh

@@ -9,4 +9,6 @@ extern fn route_imprint_user(body: String) -> String
 extern fn route_synthesize(body: String) -> String
 extern fn handle_dharma_recv(body: String) -> String
 extern fn route_sessions() -> String
+extern fn parse_session_id_from_path(path: String) -> String
+extern fn parse_session_subpath(path: String) -> String
 extern fn handle_request(method: String, path: String, body: String) -> String