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;
 

include/cst_webui.h

@@ -1,13 +1,12 @@
 #ifndef CST_WEBUI_H
 #define CST_WEBUI_H
 
-/* The built-in WebUI: an HTTP + SSE front end for one agent process.
+/* Engine HTTP + SSE API (--web): remote mirror of cst_agent_hooks_t.
  *
- * It is a second implementation of the same interface the TUI uses
- * (cst_agent_hooks_t), which is why the engine needs no changes: the turn
- * runs on a worker thread and every live event goes out as a Server-Sent
- * Event. The protocol is documented in docs/webui.md and is meant to be
- * the contract a dedicated client speaks later.  */
+ * The turn runs on a worker thread; every live hook fires a Server-Sent
+ * Event. Protocol: docs/webui.md / docs/webui-schema.md (cst-webui/1).
+ * The binary embeds a minimal health page; a full SPA lives under
+ * archive/web-spa (use --web-dev). */
 
 #include "cst_agent.h"
 
@@ -25,8 +24,8 @@ typedef struct {
   const char *host;  /* NULL = 127.0.0.1; "0.0.0.0" = every interface */
   int port;          /* 0 = pick a free port */
   const char *token; /* NULL = generate one when needed */
-  const char *root;  /* serve the UI from this directory instead of the
-                        copy embedded in the binary (development) */
+  const char *root;  /* serve static files from DIR instead of the
+                        embedded health page (development / archived SPA) */
   int open_browser;  /* best-effort xdg-open / open / start */
   /* Prompt to run once at startup (--web -p ...); NULL = wait for the UI. */
   const char *prompt;