v0.8.0: WebUI redesign, /web, workspace switch, 75% budget, no shell timeout

The front end is rebuilt on the design tokens the desktop app ships (surfaces,
color-mix borders, alpha hovers, accent): a 30-icon inline SVG sprite, dark and
light themes, layout switches as deep links (?side=0, ?panel=0, ?theme=light)
resolved before the first paint. The empty page is one line and leaves with the
first turn; the right rail keeps three cards - todo, file changes (+N -M) and
context (tokens, prompt cache hit rate). Both rails animate in and out, and a
collapsed rail widens its grid track to 0 instead of being removed, so the
transcript never slides into a zero-width column.

- src/webui.c: POST /api/workspace - re-root the agent at another directory
  (permission rules, metrics, AGENTS.md follow) and start a session there, as
  sessions live per directory; state also reports cached/cache_miss tokens
- src/agent_loop.c: cst_agent_set_workspace(), rebuilt prompt and re-measured
  budget; the auto-compact threshold is 75% of the window (was 60%)
- src/tui.c: /web forks a WebUI next to the TUI - separate process and a new
  session, because cst_webui_run() blocks and two writers corrupt one session
- src/os_compat.c, src/os_win.c: run_shell runs without a time limit unless
  CST_SHELL_TIMEOUT asks for one, so builds and long clones finish
- src/main.c, include/cst_webui.h: one CST_WEBUI_DEFAULT_PORT for --help, the
  docs and /web; Termux opens the browser through termux-open-url
- README.md, docs/webui.md, docs/env_openai.md: /web, /api/workspace, the new
  state fields, the 75% threshold and the layout deep links

tests/e2e_webui.sh green (permission round-trip, token paths, --web-dev);
make check green.

src/webui.c

@@ -181,6 +181,8 @@ static cJSON *wui_state_new(wui_t *w) {
     cJSON_AddNumberToObject(u, "requests", sess.requests);
     cJSON_AddNumberToObject(u, "prompt_tokens", (double)sess.prompt);
     cJSON_AddNumberToObject(u, "completion_tokens", (double)sess.out);
+    cJSON_AddNumberToObject(u, "cached_tokens", (double)sess.cached);
+    cJSON_AddNumberToObject(u, "cache_miss_tokens", (double)sess.miss);
     cJSON_AddNumberToObject(u, "cost", sess.cost);
     cJSON_AddBoolToObject(u, "priced", sess.priced != 0);
     cJSON_AddStringToObject(u, "currency", sess.currency);
@@ -916,6 +918,38 @@ static int h_session_new(cst_http_conn_t *c, void *ud) {
   return rc;
 }
 
+/* POST /api/workspace {"path":"/abs/dir"} - re-root the agent. The session
+ * store is per directory, so this always lands in a fresh session there. */
+static int h_workspace(cst_http_conn_t *c, void *ud) {
+  wui_t *w = ud;
+  const cst_http_req_t *req = cst_http_request(c);
+  cJSON *j = req->body_len ? cJSON_Parse(req->body) : NULL;
+  const cJSON *p = j ? cJSON_GetObjectItem(j, "path") : NULL;
+  if (!cJSON_IsString(p) || !p->valuestring[0]) {
+    cJSON_Delete(j);
+    return cst_http_send_err(c, 400, "path is required");
+  }
+  char err[256] = "";
+  int rc = cst_agent_set_workspace(w->agent, p->valuestring, err, sizeof(err));
+  cJSON_Delete(j);
+  if (rc != 0)
+    return cst_http_send_err(c, 400, err[0] ? err : "cannot switch workspace");
+  char id[128] = "";
+  if (cst_agent_new_session(w->agent, id, sizeof(id)) != 0)
+    return cst_http_send_err(c, 500, "cannot start a session there");
+  wui_emit_str(w, "session", "id", id);
+  wui_emit_state(w);
+  cJSON *o = cJSON_CreateObject();
+  if (!o)
+    return cst_http_send_err(c, 500, "out of memory");
+  cJSON_AddStringToObject(o, "id", id);
+  char *json = cJSON_PrintUnformatted(o);
+  cJSON_Delete(o);
+  rc = cst_http_send_json(c, 200, json);
+  free(json);
+  return rc;
+}
+
 static int h_session_switch(cst_http_conn_t *c, void *ud) {
   wui_t *w = ud;
   const cst_http_req_t *req = cst_http_request(c);
@@ -1134,7 +1168,10 @@ static void open_browser(const char *url) {
   os_run_shell_c(".", cmd, 0, NULL);
 #else
   char cmd[600];
-  snprintf(cmd, sizeof(cmd), "xdg-open '%s' >/dev/null 2>&1 &", url);
+  /* Termux has no xdg-open; termux-open-url hands the URL to Android. */
+  const char *opener =
+      getenv("TERMUX_VERSION") ? "termux-open-url" : "xdg-open";
+  snprintf(cmd, sizeof(cmd), "%s '%s' >/dev/null 2>&1 &", opener, url);
   os_run_shell_c(".", cmd, 0, NULL);
 #endif
   (void)url;
@@ -1173,6 +1210,7 @@ int cst_webui_run(cst_agent_t *agent, const cst_webui_opts_t *opts) {
       {"GET", "/api/sessions", h_sessions, NULL, 0, 0},
       {"POST", "/api/sessions/new", h_session_new, NULL, 0, 0},
       {"POST", "/api/sessions/switch", h_session_switch, NULL, 0, 0},
+      {"POST", "/api/workspace", h_workspace, NULL, 0, 0},
       {"POST", "/api/turn", h_turn, NULL, 0, 0},
       {"POST", "/api/cancel", h_cancel, NULL, 0, 0},
       {"POST", "/api/permission", h_permission, NULL, 0, 0},