refactor: archive GUI clients; keep minimal TUI with hooks/API

Move Qt/Tauri/full Web SPA under archive/, ship hermit as CLI/TUI plus
libcstcore and cst-webui/1 HTTP+SSE (turn lifecycle hooks, capabilities).

Co-authored-by: Cursor <cursoragent@cursor.com>

include/cst_agent.h

@@ -348,7 +348,9 @@ void cst_curl_apply_ca(void *curl);
  * detail is the tail of the sandboxed output. */
 #define CST_PERM_SANDBOX "[sandbox] "
 
-/* Live UI / observability hooks for a turn */
+/* Live UI / observability hooks for a turn. Every callback may be NULL.
+ * Remote clients prefer the HTTP/SSE mirror (docs/webui.md); embedded
+ * clients fill this struct (TUI, FFI). */
 typedef struct {
   void (*on_status)(const char *msg, void *ud);
   void (*on_assistant_delta)(const char *text, size_t len, void *ud);
@@ -375,6 +377,13 @@ typedef struct {
    * reading the store. */
   void (*on_tool_images)(const ctl_tool_call_t *call,
                          const cst_attachment_t *list, int n, void *ud);
+  /* Turn lifecycle (once around cst_agent_run_turn_ex). */
+  void (*on_turn_begin)(const char *user_text, void *ud);
+  /* rc 0 = ok; non-zero = failure or cancel. err may be NULL or "". */
+  void (*on_turn_end)(int rc, const char *err, void *ud);
+  /* Structured error while a turn runs. kind is a short stable token
+   * (llm | cancel | internal | …); message is human-readable. */
+  void (*on_error)(const char *kind, const char *message, void *ud);
   void *userdata;
 } cst_agent_hooks_t;