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

chat.elh

@@ -1,5 +1,10 @@
 // auto-generated by elc --emit-header — do not edit
 extern fn chat_default_model() -> String
+extern fn gemini_api_key() -> String
+extern fn xai_api_key() -> String
+extern fn llm_call_grok(model: String, system: String, message: String) -> String
+extern fn llm_call_gemini(model: String, system: String, message: String) -> String
+extern fn build_identity_from_graph() -> String
 extern fn engram_compile(intent: String) -> String
 extern fn json_safe(s: String) -> String
 extern fn build_system_prompt(ctx: String) -> String
@@ -12,6 +17,7 @@ extern fn handle_chat(body: String) -> String
 extern fn handle_see(body: String) -> String
 extern fn studio_tools_json() -> String
 extern fn agentic_api_key() -> String
+extern fn call_neuron_mcp(tool_name: String, args_json: String) -> String
 extern fn agentic_tools_literal() -> String
 extern fn dispatch_tool(tool_name: String, tool_input: String) -> String
 extern fn handle_chat_agentic(body: String) -> String